Stuck screw in subpanel neutral bus
Forget about the stuck screw. Go to another point in the neutral bar and remove an existing neutral. Using an appropriate wire nut or other connector connect the existing neutral with the new neutral and with a pigtail of size to carry current from both neutrals. Insert the other end of the pigtail into the neutral bar.

Before disconnecting or even loosening any neutral, turn off the breaker for that circuit.