Please check JohnKorchok's reply.
If it doesn't work, are there extra copies of your Office applications?
As far as I know, AutoUpdate may ask you over and over to update when you have more than one copy of an office app on your Mac.
Please make sure to uninstall other copies of your Office applications completely to check this issue.
Besides, I find a similar thread, you may refer to Eli Qian MSFT's reply in it.
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