12 Feb 2020: It was raised to us that OpenSolver for GoogleSheets is still unaccessible to new users. We are now able to replicate this bug. From our tests, new user accounts are able to install OpenSolver for Google Sheets as an Add-on from Chrome Web Store. However, the “Sign in with Google is temporarily disabled for this app” screen will still show up when trying to open the sidebar from the ribbon. We are very sorry for the inconvenience and we have contacted Google regarding this issue. We need to wait for Google’s response on this, therefore we are unsure of when this problem will be fixed.
Original post: Changes made by Google mean that OpenSolver for Googlesheets is currently not working for new users who want to add it to their Googlesheets. (Apparently the first 100 users each month can sign in, but then further sign ins are blocked by Google.) We are working on fixing this, But we don’t know how long it will take, sorry. We suggest you use OpenSolver for Excel until we get it working properly for Googlesheets.
Sorry for the inconvenience.