The most likely cause for this error is that the user approving the invoice is not a member of a department, so the department head cannot be identified.
The department code can be modified in Focalpoint Administrator by doing the following:
Logging into Focalpoint Administrator
Go to System Control > Users and check which Security Profile the user is in
Go to System Control > Security Profiles and highlight the Security profile that the user is in
Press Members and select the user in the left-hand list
Go down to the Department code field > Setup and add the correct department code