Hi Tim Dunn,
Welcome to the Q&A Community
I’m here to assist with your problem.
You can try some option to fix:
- Log out of your Teams account.
- Right-click the Teams icon and select Quit.
- Press the Window Key + R.
- Go to the *%AppData%\Microsoft\teams* and delete the entire folder.
- Reboot your PC and log in to Teams again.
In my test environment, I can currently "Add agenda" in both the Web App and the Desktop App. You can try again on the Web App, remembering to clear your cache and use an incognito browser window to test.
You can check your version of Teams and Update to the newest : Find the version of Microsoft Teams you're using - Microsoft Support


Reference to: Collaborative Meeting Notes on MS Teams
Take meeting notes in Microsoft Teams - Microsoft Support
We appreciate your kind patience and understanding that sometimes the initial response may not immediately resolve the issue or there may be some misunderstandings about your scenario, but we would love to hear updates from you and find out further suggestions.
Thank you for your kindness and understanding. If you need anything else, please feel free to contact me.
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