Hi. This may be a bum steer but can you confirm which OS you are working off..?
Earlier versions of Isadora were not compatible with Big Sur for Mac OS. One of the tell tale signs was that the output would be a black screen in your stage output.
If you are not running the latest public release, chances are you will need to update this to the latest version...? This is freely available from the get it part of the site.
Latest is 3.08 f12 - I think.. I know 3.07 was not friendly with Big Sur and was a black screen culprit. I got caught out with this during the week...! It took me a few minutes to remember...
Outside of that I would say you are best to post a jpeg of your screen layout and computer spec for other people to advise as to a solution.
Hope this helps.