Though user (or technician) fill all of the mandatory fields and try to submit a request, the error appear 

This error happen to all of templates and our workaround now is just to refresh the page, open the browser in incognito mode, etc. The error is random so it is hard to re-create the situation 

the Assets Number and Mobile is filled but still error

The subject field is set but error 

my servicedesk plus version is 14940