Hi Sandra,
This depends on how your Invoice screen was customized, but I can reproduce a similar issue with the default Invoice screen. The Account field is required, and the actions called by the Customer dropdown assume that the Account field has a value. For example, when you clear the Account filed and select a Customer in the default Invoice screen you get the same error message:
To resolve this, please make sure that the Account dropdown has a value before selecting a Customer.
You could also add actions in your Save & New button to automatically set the Account. To do this, in your button you would save the current Account in an action result, clear the screen, and then restore the Account from the action result. This is how the default Clear / New button works on the Invoice screen:
Action 1 saves the dropdown value, action 4 restores it. Hope this helps, please let us know if you have any further questions.
-Russell