Lucky with a bit of searching around, I found a workaround posted by Norman Parlardy:
After I copied the files, the Xojo version does build and no longer complains about UIFoundation framework missing.
Big Sur and 2019r1.1
The official line is “if you want to run Big Sur you MUST use a version newer than 2019r3.2” – at least for desktop apps (web apps arent affected in the same way)
But you CAN actually get 2019r1.1 to work for desktop apps on Big Sur
Here is what I have done to be able to run Xojo 2019r1.1 on Big Sur (Intel) – and to successfully DebugRun and Build Desktop applications:
Replace these files in the Xojo 2019r1.1 application bundle with those from 2019r3.2: