Hey there MJJ47,
We'd like to help however we can to get that prompt to not repeatedly show for entering your Exchange account password.
As you mentioned having macOS Big Sur installed, the latest version would be macOS Big Sur 11.2.2. You can verify the version number installed by clicking on the Apple logo at the top left corner of the screen, then on ‘About This Mac’. Here's how to first Back up your Mac with Time Machine, and afterwards, here's How to update the software on your Mac.
If you already have macOS 11.2.2 installed, we recommend a restart. Check out the section 'Restart' here: Log out, sleep, wake, restart, or shut down your Mac
We understand you have no issues receiving/sending emails from the Mail app with your Exchange account. Another thing to check would be if other accounts(like iCloud, Gmail, Yahoo, etc.) are having the same issue with prompting you to repeatedly enter the password. If not, that points to the Exchange account itself.
You may need to contact your Exchange account provider for further troubleshooting. A few things to ask about if you need to contact them: (account details, security details, if something needs to be done prior to setting it up on a device, special settings during setup, etc.) Exchange accounts usually involve a place of work, and may need special handling/setting up with your Mac.
Take care!