I think it's just telling you that you cannot use a semicolon (;) in timecode. if you replace that with a colon (deux points : ) it should be fine.
I just selected a range of clips in the Edit page and right clicked, chose "new compound clip" and it worked. You shouldn't need to enter a timecode, that's optional. As far as I know the only time semicolons are used in timecode is for drop-frame timecode.
EDITED to add: I see others are experiencing this problem though: viewtopic.php?f=21&t=82046
It's strange, because when I open an existing project in Resolve 15.2 I am not seeing this error; I can create a compound clip with no problems.