Before proceeding please check the following
You must be an Owner of the SharePoint site you're adding to Atvero.
The SharePoint site to add to Atvero is a Team site.
The Sharepoint site must already have the same name as the Internal Project Code of the Atvero project it will become
The SharePoint site must have an associated Microsoft 365 Group with the same name. (A Modern Team site)
If you have any questions please contact support.