To assist we will need to see your call to the endpoint, hard to check it without this.
The exact call to make is published in Atlassian's REST API documents.
To sanity check this login to the system as the user who the call's API token come from and attempt to do the action of assigning the issue manually. If you can't do it through the UI it's a permission issue and you won't be able to through Restful
Connect with like-minded Atlassian users at free events near you!Find an event
Connect with like-minded Atlassian users at free events near you!
Unfortunately there are no Community Events near you at the moment.Host an event
You're one step closer to meeting fellow Atlassian users at your local event. Learn more about Community Events