Error closing Job: Billing customer does not exist

This error usually indicates that you no longer have a customer named "Make Job". This is a special system-defined customer that must exist for proper function of the Jobs module.

All you should have to do is go into the Customer module, create a customer named "Make Job" (and use that exact capitalization) and leave it with just the default customer type.  Once that customer exists (even though you won't see it in your list) that will resolve this error with your job screen.