I took a look at the screenshot you provided, and it seems like the problem is with the mapped custom value on the “Reporter” field. From what I can see, the “Reporter” field is looking for the User ID.
Here's my suggestion: you can try adding a “Find User” action before the “Create Issue” action and map the User ID from the “Find User” step to the “Reporter” field in the “Create Issue” step. Here's what it would look like: