Error when adapting EA_Invoices to project

[Owen-Cassidy]

When I go to add invoice line I get the following error.

$form.Line_Number:=$selection_Main.max("Line_Number")+1
Process 3 Generic
Error code 15 This operation is not compatible with the two arguments.

If I continue I then get following error.

$selection_Main.add($entity)
Error code 1637 This entity selection cannot be altered.

And finally on validate I get another error.

Error when executing the method "action_Save" at line number 45
Form.displayedSelection.add(Form.editEntity)
Error code 1637 This entity selection cannot be altered.
Any help would be appreciated.