Hi
As Bob said, you should contact Grammarly for support. On their pages they show that Grammarly adds a tab to the Ribbon. I'm only taking a guess, here, and I don't have Grammarly to check this out, but here goes:
In Finder, press Shift+Command+L to display the hidden library folder.
Then follow down this path to the Word Startup items folder and see if there's a Grammarly something in there. If yes, then move it to the Mac OS trash, restart your Mac, and then empty the trash.
If you follow the link Bob provided, you'll discover Grammarly has ended support for Microsoft Word desktop and is now limiting support to the web applications.
How does Grammarly for Microsoft Word work on Macs?
https://support.grammarly.com/hc/en-us/articles/360041471771-How-does-Grammarly-for-Microsoft-Word-work-on-Macs-Important: Grammarly for Microsoft Word on Mac will no longer be updated. If you use macOS 10.14 or newer, we suggest switching to Grammarly for Mac, which supports integration with multiple applications and websites, including Microsoft Office apps. Alternatively, if you work on longer documents, we encourage you to give the Grammarly Editor a try.