Select sky

In LrC, click on Help, click on System Info, click on Copy. 

In Notepad, paste that info. then copy the section on your GPU.

Please reply with that info pasted in your reply.

Interested in this:

Displays: 1) 2560x1440
Input types: Multitouch: No, Integrated touch: No, Integrated pen: Yes, External touch: No, External pen: Yes, Keyboard: No

Graphics Processor Info: 
DirectX: NVIDIA GeForce GTX 1070 Ti (31.0.15.1694)

note that the above is dated, yours will have a bit more info. Interested in GPU make/model, and driver version

At least you can mask... I can't even upgrade past v. 11 or else it can't find the subject or the sky....

Moved to the Lightroom Classic forum from Using the Community