Error with Pages, Numbers

If updates are not showing in Update in the App Store then you probably are running

the most recent versions.

Try starting your mac in Safe Mode, this will run some disk repairs and clear to caches, it may help.

How to use safe mode on your Mac – Apple Support Safe Mode takes a long time to start up so be patient.

When booted up in Safe Mode try opening Pages and Numbers.

Then restart your mac normally.

If there is still a problem then you could also try resetting the NVRAM,

Reset NVRAM or PRAM on your Mac – Apple Support

When you upgraded to macOS Big Sur did you also update Pages and Numbers

to the versions compatible with Big Sur.

If not go to the App Store and update them.