Why a customer cannot choose a user in a user picker type custom field in the portal?

Sofia Grunspan_aup5da

In the form of the request type in the portal I have a field that is user picker (single user) type custom field but the customer cannot see the values of this field.

I've seen the field and I cannot find anything wrong with it. 

Can you help because the customer it supposed to see it.

Thanks 

Lucas Modzelewski _Lumo_

Please check this detailed answer by @Alex Koxaras -Relational- : https://community.atlassian.com/t5/Jira-Service-Management/User-picker-does-not-populate-for-ServiceDesk-customers/qaq-p/1831778