Customised multi select field is getting error message

Summary:

I have created a customised multi select field called Request Category. It is being used in HR Help Desk Redwood. When i select more than one value I keep getting this error message. Please assist me with what the error message means:

Content (please ensure you mask any confidential information):

Version (include the version you are using, if applicable):

Code Snippet (add any code snippets that support your topic, if applicable):