Resolved by following https://support.office.com/en-us/article/Office-repeatedly-prompts-you-to-activate-on-a-new-PC-a9a6b05f-f6ce-4d1f-8d49-eb5007b64ba1
Close the activation window and all Office apps.
- Right-click the Start button on the lower-left corner of your screen, and select Run.
- Type regedit, and then press Enter. Select Yes when prompted to open the Registry Editor.
- On the left side of the Registry Editor, under Computer, navigate to the following key in the registry:
- Right click the OEM value and click File>Export.
- Save the key.
- After the key is backed up, select Edit>Delete.
- Repeat steps 3-6 for the following key:
- Close the Registry Editor and start Office again.