[ Liberate ] Cells do not have -extsim_model parameter specified in define_leafcell command

I am trying to characterize a standard cell library, but liberate gives me the following errors.

WARNING (LIB-40): (set_var): Ignored the invalid value '$model' specified for 'extsim_model_include'. It should be set to an absolute file path.
INFO (LIB-511): (define_leafcell): Leafcell 'N_18_MM' (instance) has been identified with pin_position (0 1 2 3) mapped to (D G S B).
LIBERATE parameter "extsim_exclusive" set to "1"
INFO (LIB-511): (define_leafcell): Leafcell 'P_18_MM' (instance) has been identified with pin_position (0 1 2 3) mapped to (D G S B).
LIBERATE parameter "simulator" set to "spectre"
LIBERATE parameter "char_library_skip_var_list" set to ""
Start Characterizing Library at (Wed May 19 00:36:13 IST 2021)

WARNING (LIB-909): (read_spice): Could not find a model/subckt definition for instance 'c' named 'mim_va' with '2' terminals.  Liberate will attempt to guess the device type. Read the models in read_spice or use define_leafcell to map the name to a model/subckt and rerun.
WARNING (LIB-909): (read_spice): Could not find a model/subckt definition for instance 'r1' named 'res_va' with '2' terminals.  Liberate will attempt to guess the device type. Read the models in read_spice or use define_leafcell to map the name to a model/subckt and rerun.
WARNING (LIB-909): (read_spice): Could not find a model/subckt definition for instance 'rs' named 'reshr_va' with '2' terminals.  Liberate will attempt to guess the device type. Read the models in read_spice or use define_leafcell to map the name to a model/subckt and rerun.
WARNING (LIB-909): (read_spice): Could not find a model/subckt definition for instance 'rs' named 'resnp_va' with '2' terminals.  Liberate will attempt to guess the device type. Read the models in read_spice or use define_leafcell to map the name to a model/subckt and rerun.
WARNING (LIB-909): (read_spice): Could not find a model/subckt definition for instance 'rs' named 'respp_va' with '2' terminals.  Liberate will attempt to guess the device type. Read the models in read_spice or use define_leafcell to map the name to a model/subckt and rerun.
WARNING (LIB-933): To enable automatic leaf-cell recognition, the variable 'extsim_model_include' is required.
INFO (LIB-956): (read_spice): Reading file: 'dut.scs'.
INFO (LIB-955): (read_spice): Further occurrences of the preceding message will be suppressed.
INFO (LIB-940): The parser has identified the following leaf cells. Review these for missing or incorrect settings and if needed, add them to your Tcl script and rerun.
INFO (LIB-906): (AUTO): define_leafcell -type black_box -pin_position {0 1 2} l_slcr20k_rf
INFO (LIB-906): (AUTO): define_leafcell -type c -pin_position {0 1 2} mimcapm_rf
INFO (LIB-906): (AUTO): define_leafcell -type c -pin_position {0 1} mimcaps_mm
INFO (LIB-906): (AUTO): define_leafcell -type nmos -pin_position {0 1 2 3} n_l18w500_18_rf
INFO (LIB-906): (AUTO): define_leafcell -type nmos -pin_position {0 1 2 3} n_l34w500_33_rf
INFO (LIB-906): (AUTO): define_leafcell -type nmos -pin_position {0 1 2 3} n_po7w500_18_rf
INFO (LIB-906): (AUTO): define_leafcell -type nmos -pin_position {0 1 2 3} n_po7w500_33_rf
INFO (LIB-906): (AUTO): define_leafcell -type pmos -pin_position {0 1 2 3} p_l18w500_18_rf
INFO (LIB-906): (AUTO): define_leafcell -type pmos -pin_position {0 1 2 3} p_l34w500_33_rf
INFO (LIB-906): (AUTO): define_leafcell -type pmos -pin_position {0 1 2 3} p_po7w500_18_rf
INFO (LIB-906): (AUTO): define_leafcell -type pmos -pin_position {0 1 2 3} p_po7w500_33_rf
INFO (LIB-906): (AUTO): define_leafcell -type diode -pin_position {0 1} vardiop_rf
INFO (LIB-906): (AUTO): define_leafcell -type black_box -pin_position {0 1 2} varmis_18_rf
INFO (LIB-907): (AUTO): define_leafcell -element -type c -pin_position {0 1} mim_va
INFO (LIB-943): Finished reading netlist(s) at May 19 00:36:13.
INFO (LIB-711): Feature 'Virtuoso_Multi_mode_Simulation' exists in the license pool. The parameter 'spectre_use_mmsim_token_license' will be set to '1'.
INFO (LIB-1008): (char_library): This LIBERATE release was qualified with MMSIM version '' but newer version '19.1.0.396.isr8' was detected. If MMSIM-related issues are found, update to the qualified MMSIM version and re-run.
INFO (LIB-966): Using Spectre version 19.1.0.396.isr8 located at: /home/installs/SPECTRE191/tools/bin/spectre.
*Info* Use temporary directory '/home/TSMC/Desktop/UMC180/Designkits/Cadence_6.1/lowpower/liberate'.
LIBERATE parameter "extsim_deck_dir" defaulted to cad19:/home/TSMC/Desktop/UMC180/Designkits/Cadence_6.1/lowpower/liberate/decks.cad19.T20210519003610741304S0014229
*Error* (char_library) : set_var extsim_use_leaf_cell is enabled, but
 not all leaf_cells have -extsim_model parameter defined. Since set_var extsim_model_include
 is not used.  Cannot continue, exiting.
*Note* : The following cells do not have -extsim_model parameter specified in define_leafcell command.
      : N_18_MM
      : P_18_MM

Peak memory usage:          340 MB
Peak virtual memory usage:  303 MB
Peak physical memory usage: 37 MB
Wall time      :    0.00 hours (3.00 seconds)
LIBERATE exited on cad19 at Wed May 19 00:36:13 2021

My char.tcl file is as follows:

define_template -type delay -index_1 {61.75 158.9 794.1} -index_2 {18.732 37.464 74.928} delay_3x3
define_template -type power -index_1 {61.75 158.9 794.1} -index_2 {18.732 37.464 74.928} power_3x3
define_template -type constraint -index_1 {61.75 158.9 794.1} -index_2 {18.732 37.464 74.928} constraint_3x3

set model "/home/TSMC/Desktop/UMC180/Designkits/Cadence_6.1/Models/Spectre/core_rf_v2d4.lib.scs"

set_operating_condition -voltage 0.3 -temp 25

set_var extsim_model_include \$model

define_leafcell -type nmos -pin_position {0 1 2 3} N_18_MM
define_leafcell -type pmos -pin_position {0 1 2 3} P_18_MM

read_spice -format spectre {dut.scs}

define_cell \
-input {in} \
-output {out} \
-delay delay_3x3 \
-power power_3x3 \
-constraint constraint_3x3 \
{INVX1}

define_cell \
-input {a b} \
-output {y} \
-delay delay_3x3 \
-power power_3x3 \
-constraint constraint_3x3 \
{NAND2X1 NOR2X1 AND2X1 OR2X1 XOR2X1 XNOR2X1}

char_library -extsim spectre
write_ldb lowpower.ldb
write_library lowpower.lib

I am using UMC's 180nm FDK and the MOS devices are named as N_18_MM and P_18_MM for nMOS and pMOS respectively.

I have a feeling that there's something wrong with my .tcl file

How do I go about this?

Thanks in advance!