See the article Limits for iCloud Contacts, Calendars, Reminders, Bookmarks, and Maps - Apple Support
Possibility calendar application is corrupted , create a new admin account in users and groups Set up users, guests and groups on Mac - Apple Support
New admin account for mac os catalina 10.15.7 when two factor authentication is used for apple id account .
( a ) In mac system preferences open users and groups , unlock the lock using admin password click on plus sign in new account choose as admin structure , fill up the credentials for new name and new password , quit system preferences , the pad lock will automatically get locked .
( b ) Take cursor on top menu bar and click on apple logo and log out the primary admin account , then on the pop up screen appeared on the desktop click on log out .
( c ) Sign in with new admin name and its password .
( d ) A screen appears for data and privacy click on right arrow of continue .
( e ) Sign in with your apple id and click on continue , enter the apple id and password and click on continue a small circle rotates setting up account .
( f ) Enter the password you previously used to unlock the mac book , your password is encrypted and cannot be read by apple , enter old admin account password and click on continue a small circle rotates .
( g ) Check the box of share crash and usage data with app developers and click on continue .
( h ) A screen appears for screen time , click on continue .
( i ) The box of siri is enabled by default , click on continue .
( j ) The box of ( 1 ) store files from documents and desktops in icloud drive ( 2 ) Store photos and videos in icloud photos are checked by default , click on continue and choose your look select any dark mark / light mode it will show setting up mac , three right check mark will appear .
( k ) On the top right corner notification will appear google account added ( it depends which mail services is use yahoo , hotmail ….) click on turn on a pop screen appears enter the password for gmail account click on open safari a window for accounts related with gmail will open , the web gmail address is already saved as highlighted , click on next and enter gmail password and click on next the gmail account is configured in interenet accounts , quit system preferences and close gmail window in the background .
( l ) Open system preferences click on accessibility again click on pointer control trackpad options click the box of enable dragging using three finger drag and click on ok , click on display check the box of increase contrast the reduce transparency box will be dimmed .
( m ) Click on mail application the mails will start syncing take cursor on view and click on mail box list , click on photos application it show what’s new in photo click on get started the photos will start syncing , click on calendar it will show what’s new in calendar click on continue and showing you can configure features in preferences , click on view show calendar list , click on notes it will show welcome to notes and click on continue open its preferences you can drag the pointer to extreme right for changing text size , same way for reminders click on continue .
And once again configure calendar account Add or delete calendar accounts on Mac - Apple Support