Mac App Store: We could not complete your purchase - Unknown Error

Quit your App Store application if it is open. Launch the terminal application and type the following command:

open $TMPDIR/../C/com.apple.appstore/

A folder named com.apple.appstore will open. Move all the files present in it to Trash. Restart your mac and launch the App Store again. Problem solved!

Removing contents of /var/folders and /private/var/folders cache directories fixed this issue for me. Open your terminal and run

sudo rm -fr /var/folders/**
sudo rm -fr /private/var/folders/**

Found this solution there

Do not be afraid to break something by removing contents from /var/folders and /private/var/folders because they are used to store per-user temporary files and caches

The reason for this is that:

Your Xcode application was installed previously with a different Apple ID. I would suggest you to check if there is any account active still in @Internet Accounts. Remove it and also go to Profiles and check if there is MDM profile installed or not. If you have an MDM profile, you have to reach out your IT to fix it.

Also, sign out from Mac Store and iTunes, reboot and try to sign in back if there is no MDM profile installed on your Macbook.