Buck converter in pspice output value changes depending on the resistance. How do I correct this?
You have no control loop around your buck converter.  To a first order, IF the inductor current is continuous the output voltage will be the duty cycle times the input voltage.
However, DCR in your inductor and RDSon for your FET as well as your diode's forward drop and incremental resistance will cause errors.
A typical buck converter has a circuit to measure the output voltage, compare it to a reference proportional to the desired voltage, and correct the duty cycle to keep the output constant with load and input voltage variations.
This circuit requires control loop compensation, and could include an inner current feedback loop.  Without feedback you would expect the output voltage to change with load.
Here's an example block diagram: