From CAESAR II UG,

WRC 107 :"....used to compute the stresses at both the inner and outer surfaces of the vessel wall,...."

WRC 297:"...WRC 297 also computes stresses in the nozzle and the vessel. (WRC 107 only computes stresses in the vessel.)..."
CAESAR II calculates the stress components at the four locations on the vessel around the nozzle and also the corresponding locations on the nozzle.

If you have modeled Nozzle as WRC 297 that means you have considered nozzle to be flexible. Hence WRC 107 cannot be applied (WRC 107 implies nozzle is rigid).

You should understand the error report generated upon selecting nozzle as WRC 297 shows nozzle flexibility (finite stiffnesses) not stresses.