office 365 premium will not install or run any third party add-ins, but native work (like copilot)?

Hello Simon Allen,

Since Copilot works but third-party Excel add-ins do not, this does not necessarily mean that Microsoft 365 Premium is blocking third-party add-ins.

A previous work or school account, Office privacy settings, an add-in security setting, or an authentication problem could still affect how Office loads them.

To narrow this down, could you provide:

1. Are you using Windows or macOS?
2. In Excel, what version and build are shown under File > Account > About Excel?
3. What happens when you install or open an add-in? Please include the exact error message.
4. Does this affect every third-party add-in or only Claude?
5. Does the same add-in work in Excel for the web?
6. Under File > Account, do you still see any old work or school accounts under signed-in accounts or connected services?

Meanwhile, please check the following:

1. Confirm that the computer’s date, time, and time zone are correct, as incorrect settings can prevent Office from validating add-ins.
2. In Excel, select File > Account > Update Options > Update Now.
3. Select File > Options > Trust Center > Trust Center Settings > Add-ins, and check whether application add-ins are disabled or restricted.
4. Sign out of all accounts in Excel, close every Office app, restart the computer, and then sign back in using only the Microsoft account that owns the Microsoft 365 Premium subscription.
5. Test the add-in in a new blank workbook and in Excel for the web.

If third-party add-ins also fail in Excel for the web, the issue is less likely to be caused by the local Office installation. In that case, please provide the exact error and the results of the checks above before reinstalling Office again.

Feel free reply back to this post and update me on this issue, I'll be happy to help.