Multiple users from a group/list select once creating a custom field

[Bahar Dehkordi]

I am using 'Jira utilities' add-on, so that I can choose multiple users from a list/ group (I do not want to choose from all users) once creating a custom field.
After installing this ad-on it gives me the option, and I can choose the target users, but once I am going to use the custom field in a customer panel, it does not show it at all because the field's option is empty!
I receive this error:
"Request Form contains configuration errors
The following fields don't have any values set:
approvers test
As a consequence these Request Forms cannot be used successfully by customers:
Travel & Event Planning Document
To repair these Request Forms, add values for these fields on this Configure Custom Field page."
And once I click on it, I see the screen shot. It is like the options that I created in this custom field get removed automatically. 

Any help, hints is much appreciated.

Thanks,

Bahar