Free Math Editor to build equation and insert into Google Docs documents.
For shared documents the 'Google Add-on' is installed but is not authenticated. The solution is to reinstall again from Google Marketplace.
1. Install the extension from Google Marketplace(menu: "Add-ons" / "Get add-on").
2. Click to "Add".
3. Give the permission to run
4. Instalation done
Go to Support Page