Here is how to troubleshoot this:
Step 1:  Do you have other interpreters who are able to successfully log in to the mobile app?  If no, then contact LSP Ware at
Step 2:  Is this interpreter able to log in to the Interpreter Portal?  If no, then there is a good chance they do not know their password.  Reset their password and then follow the steps below to sync the passwords.
Step 3:  Are they authorized for the Mobile App?  Go to Administration/Maintain Users and locate this interpreter.  Make sure the Mobile Access check box is selected.  If it is not, then select it and click Save.  (You may have to make an edit to one of the other fields to get enable the Save button).  That should solve the issue.