Can't create an unavailable time signature.
In Musescore 3, there was a "more" option under the time signature palette that allowed me to create uncontemporary time signatures - this doesn't show anywhere in Musescore 4 and the only edit option for time signatures changes the text display only and doesn't effect playback.
Comments
There should be "More" entry in the palette. Is the palette enabled for editing? Click on the three dots to the right if the palette name to check.
If all is ok there, you could try Help>Revert to factory settings but note that this will reset ALL customisations so save any custom palettes, styles shortcuts that you don't want to lose first.