Xojo green "RUN" icon is not enabled

I have updated to 2020 R3.1 and when I load a project, the Xojo green “RUN” icon is not enabled? what have I done wrong? This means I am stuck with some large projects not being accessible for development and testing. Please help.

Which version ?
2020r3.1 doesnt exist
Do you mean 2019r3.1 ?

often when an old project os loaded the run button is disable because its set to run as 32 bit on a machine that only runs 64 bit code (macOS specifically)

Click on the macOS item in the navigator and see if thats the case

Thank you for your prompt response. You are absolutely correct. Using MAC OS Catalina 10.15.4 means 64-bit. Brilliant it now works.