a few months ago, last year in fact, The developers of ProRender made only the RX (400 series and newer) GPUs and newer GPUs compatible with ProRender.
Your GPU isn't compatible anymore even though it may still work. Depending on the Non-RX GPU and Prorender version being used.
Reason why I know because another User was having ProRender Plugin issues with his R9 GPU last year and I suggested he open a thread at ProRender Forum. One of the Developers of ProRender advised him that all Non-RX GPUs are now incompatible even though some may work still.
The Manager of ProRender then updated the Compatibility List for ProRender excluding all Non-RX GPUs from the list.
I opened a thread letting other Users know about this change in ProRender compatibility list.
EDIT: The second link (https://www.amd.com/en/support/kb/release-notes/rn-prorender-3dsmax-v2-7) was the list that the developer of ProRender changed and updated and eliminated the R9 Series GPU which was listed before.