I purchased Microsoft 365. It says it was installed, but it does not activate. It comes in with an unknown error message.

Are you running Tahoe macOS? If so, make sure all updates on the macOS side is caught up. That alone might be the issue.

Otherwise,

First, see if you can update your Office apps. Open Word for example, click on the 'Help' tab >Check for Updates. Make sure you choose Update All when prompted. Let the process run through, can take a bit time. See if that works.

Otherwise, use the Update method in this article: https://support.microsoft.com/support/known-issues/unknown-error-0x0-license-not-found-error-microsoft365-macos?apid=ets

If it still doesn't work, please uninstall the apps. Follow the instructions here for complete uninstall.

https://support.microsoft.com/office/office-for-mac/troubleshoot-office-for-mac-issues-by-completely-uninstalling-before-you-reinstall?apid=ets

Then reinstall this way:

If you signed up via Apple App Store, then download and install from the App Store.

If you signed up via Microsoft directly, please go to https://www.office.com

Sign into the Microsoft account you used for the subscription. Click on the 9-dot icon on the upper left > More apps.

Then on the upper right, click on "Install apps". This will now download the installer file. Once downloaded, click on the file to install. It will install the Desktop version of the apps for you then.

https://support.microsoft.com/office/download-install-or-reinstall-microsoft-365-or-office-2024-on-a-pc-or-mac-4414eaaf-0478-48be-9c42-23adc4716658?apid=ets

If it still doesn't work, download the updater https://support.microsoft.com/office/office-for-mac/troubleshoot-microsoft-autoupdate-in-microsoft-365-and-office-for-mac?apid=ets

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If this is not on Tahoe, please tell me what OS do you have.