Unfortunately, we've recently been notified that Grammarly for Windows and Mac was deactivated on Atticus (not by us!) due to compatibility issues.
At this time, we don’t have any information about whether or not they will be resolving the issues, but it's out of our ability to control. For the time being though, we recommend turning it off when you're using Atticus.
We understand that this important for your workflow, so a work around would be to export your manuscript to Word for the editing process. Once you've finished that, you can re-upload manuscript into Atticus for formatting. Be sure to the follow the guidelines for how to format your .docx properly, which you can find here: How to Prepare Your Word Document for Upload.
Alternatively, we do have a great relationship with the team at ProWritingAid, and know their extension works well with Atticus! If you're interested in checking that option out, we do recommend it highly. You can check out their features here: https://www.atticus.io/prowritingaid
We hope this helps!