Processes in Microsoft 365 for setting up Office apps, redeeming product keys, and activating licenses.
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Please check this:
Open Word > File > Account. Look under Product Information. Does it say Product Activated, Subscription Product, Unlicensed Product, Product Activation Failed, or View Only?
If you see an activation or license problem, try this: Sign out of Word, close all Office apps, then open Word again and sign back in with the Microsoft account that owns your Office license.
If there is an Update License button on the File > Account page, click that.
You can also repair Office:
Windows Settings > Apps > Installed apps > Microsoft 365 or Microsoft Office > Modify > Online Repair.
An old version of Office normally should not lock all documents by itself, but if the old Office install is no longer activated or is not linked to a valid license, Word can block editing.
What does Word show under File > Account > Product Information?