Hi @Marc ten Cate ,
According to this documentation, We can fetch permissions on a SharePoint site and not for a subsite. As this webUrl https://TENANT.sharepoint.com/SITENAME represents a sub-site within root site collection that is why you are getting operation not allowed error. I am also able to replicate this scenario.
You can submit a feature request idea which will be monitored by Microsoft team and make the enhancements to Microsoft Graph APIs.
Hope this helps.
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