Cannot load symbol

Look at the PCB Footprint property for this part. It looks like you have called this 806 and a footprint of that name doesn't exist in the required directories. An IRF150 would typically be a TO204AE and a footprint of this name does exist in the default library C:\Cadence\SPB_17.4\share\pcb\pcb_lib\symbols folder. Try changing the footprint to that or the part you actually want to use.

I tried what you said. I changed the part to the one with pcb footprint TO204AE but again it shows up the message : Cannot load symbol 'TO-204AE'. I checked the default librabry you mentioned and the name exists in there.

So the PCB Footprint has 4 pins, what does your schematic symbol have? If these don't match you'll get the error. Try opening the PCB Footprint and you'll see 2 pins for the can connection (pins 3 and 4). If your schematic symbol only has 3 pins add a new property called NC with a value of the non connect pins (4). Try watching this video OrCAD Simple PCB Design Tutorial 17.4 - YouTube which goes through a simple flow (and shows the property being added).

Indeed the schematic symbol has 3 pins. Before i go on i want to mention that using the IRF150 with the PCB footprint name TO204AE and simulating the circuit i dont get the response that i get using the IRF150 with the 860 PCB footprint name . Is there a problem or it will be ok even if i use TO204AE footprint?

The PCB Footprint property has nothing to do with the simulation. You may need to explain what you mean by I don't get the response?

While designing  the schematic of my circuit (before PCB design) PSpice has multiple choices for the transistor IRF150 that i want to place . One  of them has PCB footprint name 860 (and when i simulate my circuit i take the desired response,that is the voltage i want at the output) and another choice has PCB footprint name ,as you told me , TO204AE (but the simulation results at time domain arent the desired). I asked if there is a problem if i use the TO204AE for the PCB design even thogh the simulations of PSpice are not the desired

Check whether you have provided the padpath and psmpath (path where the PCB footprints are located) in user preferences of PCB editor

Check whether you have provided the padpath and psmpath (path where the PCB footprints are located) in user preferences of PCB editor