If you are unable to install and Google Workspace add-on, or if the Install button itself is greyed and disabled, you need to contact your domain admin and request them to whitelist the addon.
Open admin.google.com to access your Google Workspace domain admin panel.
Go to Apps > Google Workspace Marketplace Apps > Apps List
You may select Everyone to allowlist the selected addon for an organizational unit or specific groups. To add other groups or organizational units, repeat the steps.