If you are unable to add new contacts on your Mac running Big Sur, there are a few steps you can try to resolve the issue:
Check your iCloud settings: Make sure that your iCloud account is properly set up on your Mac and that your Contacts app is syncing with iCloud. You can check this by going to System Preferences > Apple ID > iCloud and making sure that Contacts is enabled.
Restart the Contacts app: Sometimes, simply restarting the Contacts app can resolve the issue. To do this, quit the app by clicking on Contacts in the menu bar and selecting Quit Contacts. Then, reopen the app.
Clear your cache: Clearing the cache of the Contacts app can sometimes resolve issues with adding new contacts. To do this, go to Finder > Go > Go to Folder and type in ~/Library/Caches/com.apple.Contacts/. Then, select all the files in this folder and move them to the Trash. Restart the Contacts app and try adding a new contact again.
Check for software updates: Make sure that your Mac is running the latest version of Big Sur and that all updates have been installed. To check for updates, go to System Preferences > Software Update.
Reset your contacts: If none of the above steps work, you can try resetting your contacts. This will delete all your contacts and then restore them from your iCloud account. To do this, go to Contacts > Preferences > Accounts, select your iCloud account, and click on the “Reset Contacts” button.
If you still cannot add new contacts after trying these steps, you may want to contact Apple Support for further assistance.