It's in the Windows menu. You also need to have the focus on a Bin for it to be available.

I initially had the same problem you describe with 2021.6. But since fiddling with that Interface setting it has come good, so I think the two are mysteriously related...

Thre has been a couple of changes to the way Workspaces behave in 2021.6 which have caused this. Firstly, "clicking away" from an effect - onto the TC track, for example - now closes the Effect Editor in the Effects workspace, whereas it used to leave it open but go blank. You are, however, still in the Effects Workspace. This then combines

I presume the artefacting only shows up once the Timewarp has been rendered - and not while it's still green dot? In which case, what codec do you have set for rendering in your Media Creation settings?

I agree with these comments too! Sorry for not mentioning these things. The OP was asking about potential advantages of Catalina or Big Sur. I therefore presumed they were aware of the disadvantages..

None that I know of!

Ah, update! I also tried this on my Big Sur system. I got the same error message you did. Changing permissions had no effect. So this may be OS dependent...

I just tried tried this on my Mojave system. It worked fine to save in the EQ 3.0 folder, or into a new 'Mike' sub-folder I created at that level, but saving to another location - eg Desktop - triggered the error. Have you checked permissions on the EQ3.0 folder? Mine is set r/w for my user, read-only for everyone else.

Right-click the sequence in the bin and choose Commit Multicam from the menu. It'll make you a 'flattened' copy that is linked to the individual source cllips rather than the groups. HTH

I've found the reliability/stability of MC on Big Sur to be better than Catalina generally. But Mojave is still to be preferred if you want the most flexibility of codec support and import options, not to mention title tools/