Running in Loops
If you press the "Install" button, it shows a loading circle on a white window, but doesn't move on to 'accepted' window.
You can resolve this issue by signing out of multiple Google accounts.
Install Button Disabled
If you are unable to add the add-on to your Google Sheets because the install button is disabled and grayed out, you need to contact your Google Workspace admin to whitelist the app.
Please see this page detailing how your admin can add the Mail Merge app to your domain's allowlist.