Am using audacity 3.0.0 in ubuntu 21.04 and this error appears every time I try to open a project of mine. Already tried to install the latest versions, restart configurations, re-installing audacity etc. But nothing
Until someone who actually knows how the program works comes around 
some questions from me:
How did you install - appimage? repository? another way?
And is there a reason why you’re using an older version, 3.0.0? 3.1.2 is available.
Does the error appear if you create and save (and try to reopen) a project all within this version?
What version were your projects created in? If v2.x - Audacity 3 has a different format.