Of course you did not give your service principal or managed identity Sites.ReadWrite.All right? Right?
Ok, well, if you did, I can imagine, since there is no GUI for it and having to look up site ID’s, MI/SPN’s GUID’s and then executing Graph queries is quite the hassle!
So I added a small feature to SPNRoleMgr that makes this a 100% GUI workflow directly in your browser (no installs, no dependencies!).

/tools was built mainly for myself, but you are more than welcome to use it to make your life easier (and please get rid of sites.readwrite.all!)

