First some clarification. Are you leveraging this servicenow integration using SIM or SDIM?
I actually had this the other day, and discovered that tenant had a legacy SIM integration defined. SIM was replaced by SDIM.
Using the get-sim-integrations API endpoint, check and see if you have any defined, and if any of the sources you’re trying to provision to are listed in its source ids.
The sailpoint support helped disable that attribute from their end for our tenant. After that it is no more asking for that and that resolved the issue.