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Hi, Leah Ross
This usually happens when Word’s Editor/Similarity service cannot connect properly, the feature is being opened from the wrong version of Word, or Word has a local add-in/cache issue. Similarity is reached from Home > Editor > Similarity > Check for similarity.
Here are some suggestions you can try:
Use the Word desktop app, not Word for the web. Similarity Checker was disabled in Word Web, so the button may not appear or may fail there.
Open Word, go to File > Account > Update Options > Update Now, then restart Word.
Sign out of Word, close all Office apps, reopen Word, and sign back in with the Microsoft 365 account that has the subscription.
Try Word in Safe Mode: press Windows + R, type winword /safe, and test Similarity again. If it works there, disable add-ins under File > Options > Add-ins > COM Add-ins > Go.
If it still fails, run an Office repair from Windows Settings > Apps > Microsoft 365 > Modify > Quick Repair, then try Online Repair if needed. It is recommended that you also include your Word version and a screenshot if you contact Microsoft Support.
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Thank you for your patience in reading, I hope this information has been helpful to you.
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