As it says on the manual page (click the “?” button at the bottom of the error message to see this page: https://alphamanual.audacityteam.org/man/Error:_Audacity_settings_file_unwritable)…
Check that the file:
/Library/Application Support/audacity
can be written to.
An alternative fix is given by Bill here: https://forum.audacityteam.org/t/audacity-configuration-error/62320/6