The most commonly reported issue by far is that the sidebar doesn't load within Google Sheets.
Here are the two reasons why this might happen.
Almost all users who report that the sidebar isn't loading have the same reason: they're logged into multiple Google accounts in their browser.
Developers have been dealing with this issue since Google Workspace was first released, with the first ticket created in 2017. For an explanation and solutions, see the documentation on the multiple Google accounts detected issue.
If you have a slow internet connection, you may need to wait some time before any extensions load. This is a shortcoming of how Google Sheets work, and applies to any extensions that you have installed.
Google Workspace does not share the contact details of any reports made through their platform:
As such, contact us via live chat if you're still running into issues.