As of November 29, 2023, to upload a document in Turnitin, a student must be registered in that class and the upload must be made on the student's behalf. You can find Turnitin's official announcement about this feature at this link.
When it comes to theses, register students on Turnitin and upload on behalf of the students.
Additionally, "Quick Submit" can be used as an alternative upload method, but it is important to select "no repository" as you move towards uploading the document. You can find detailed information about the "Quick Submit" feature in the links below: