https://windowsaddict.ml/office-license-is-not-genuine



If you are seeing this banner on Office apps, the below activators can resolve it.

Use the latest KMS_VL_ALL (Mirror) or MAS to activate Office
Done.
 

Alternate Solution:
Change the Office C2R update channel to Production::MEC | MonthlyEnterprise
To do that, open the command prompt as admin and enter below lines and press enter,
reg add HKLM\SOFTWARE\Microsoft\Office\ClickToRun\Configuration /v CDNBaseUrl /t REG_SZ /d "http://officecdn.microsoft.com/pr/55336B82-A18D-4DD6-B5F6-9E5095C314A6" /f
reg delete HKLM\SOFTWARE\Microsoft\Office\ClickToRun\Configuration /v UpdateUrl /f
reg delete HKLM\SOFTWARE\Microsoft\Office\ClickToRun\Configuration /v UpdateToVersion /f
reg delete HKLM\SOFTWARE\Microsoft\Office\ClickToRun\Updates /v UpdateToVersion /f
reg delete HKLM\SOFTWARE\Policies\Microsoft\Office\16.0\Common\OfficeUpdate\ /f
"%CommonProgramFiles%\microsoft shared\ClickToRun\OfficeC2RClient.exe" /update user
Make sure the Internet is connected and Office is successfully updated with the last command.
Source, thanks to @abbodi1406
Thanks to @Teknik for the idea of changing the update channel.