But even though you have a premium account and tried to log in and nothing happens, then, you just need the fix we have here.
I am currently using a Premium Grammarly account, and I did a simple fix for this error. See the steps below:
Grammarly is active, but key features are missing Chrome extension.
1. Go to Chrome Menu (3 dots) > More Tools > Extensions.
2. Find Grammarly and click Details.
3. Find Site access > and select On all sites.
4. Close Chrome and open again.
5. Check again, and you will see that the "Grammarly is active, but key features are missing" is now gone.
If you want a free Premium Grammarly account, then follow the tutorial below.
Get a free Grammarly premium account.
Now, if all the fixes above do not work, you can try to remove the Grammarly extension and add it again to your Chrome browser.