I see others have similar issues. here is my setup:
Model: iMac19,1, BootROM 1037.80.34.0.0, 8 processors, 8-Core Intel Core i9, 3.6 GHz, 32 GB, SMC 2.46f12
Graphics: Radeon Pro Vega 48
I am using latest version (2.81a) of Blender.
I have tried just ticking GPu only but that doesn't help.
Any help greatly appreciated as need to be able to use my expensive iMac with blender and get rendering!
BTW Before I installed ProRender for Vega things worked fine but could only use CPU as Apple removed support for anything but Metal.
The Apple community would be a better place to ask as there are lots of experienced people there
Official Apple Support Community