How to solve this problem in simulation?

Your function is likely invalid. "^" in PSpice expressions is Boolean XOR, I suspect that you actually need PWR, or PWRS, to raise a variable by a power. (See the PSpice Users Guide) Also, your Csc function has an X parameter "passed in" but X isn't used anywhere in the function expression.

I modified the model based on your advice:

* PSpice Model Editor - Version 17.2.0
.SUBCKT Cj NP NN
.PARAM C0=2.1E-13
.PARAM V0=2.7
.PARAM a=0.7
.PARAM ISS=56.19E-6
.PARAM Vb={a*V0}
.FUNC Csc(X) {IF(X<Vb,C0/SQRT(1-X/V0),C0*SQRT((1-a)*(1-a)*(1-a))*(1-1.5*a+0.5*X/V0)}
***** BEGIN CIRCUIT *****
*SCH SECTION
.MODEL DSCH D (IS={ISS} N=2 RS=0.45)
DS NP NN DSCH
GCsc NP NN VALUE={Csc(V(NP)-V(NN))*DDT(V(NP)-V(NN))}
.ENDS

But the result still the same:

**** FROM LIBRARY ../../../cj.lib ****
.SUBCKT Cj NP NN
.PARAM C0=2.1E-13
.PARAM V0=2.7
.PARAM a=0.7
.PARAM ISS=56.19E-6
.PARAM Vb={a*V0}
.FUNC Csc(X) {IF(X<Vb,C0/SQRT(1-X/V0),C0*SQRT((1-a)*(1-a)*(1-a))*(1-1.5*a+0.5*X/V0)}
***** BEGIN CIRCUIT *****
*SCH SECTION
-------------$
ERROR(ORPSIM-16367): End of expression not seen
DS NP NN DSCH
------------------------------------$
ERROR(ORPSIM-16367): End of expression not seen
DS NP NN DSCH
------------------------------------$
ERROR(ORPSIM-16367): End of expression not seen
GCsc NP NN VALUE={Csc(V(NP)-V(NN))*DDT(V(NP)-V(NN))}
------------------------------------$
ERROR(ORPSIM-16367): End of expression not seen
.ENDS
*
------------------------------------$
ERROR(ORPSIM-16499): Missing .ENDS in .SUBCKT

I did put .ENDS in the end of the model and the expression is complete. Could you help me with this problem?

I modified the model base on your advice.

.FUNC Csc(X) {IF(X<Vb,C0/SQRT(1-X/V0),C0*SQRT((1-a)*(1-a)*(1-a))*(1-1.5*a+0.5*X/V0)}

But the simulation result is exactly the same.

I did put an .ENDS in the end of the model and the expression is complete. I don't understand why the errors keep show up. Could you please help me with the problem?

The IF statement is missing a closing bracket at the right hand side.