If you are not able to activate your office for Mac, here are a few things you can try to activate it.
Make sure you have a genuine copy of Office 365 from the official website.
Then reset the credentials and remove the key chain using these steps.
Open Finder and click on the applications button
Now, select utilities and ten keychain access and remove these password entries.
Microsoft Office Identities Cache 2
Microsoft Office Identities Settings 2
Then, you need to connect to a good internet connection.
You should also make sure that you are using the administrator account of the Mac.
Follow all this and your problem will be resolved pretty easily.
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