When invoicing a rental leasing agreement on a new corporation, or a corporation new to rental leasing, there's a chance it does not have the correct facility types set.
On Fleet > Rental Manager > To Be Invoiced when you're attempting to invoice an agreement you might see the following error,

Error: Error creating record in
op_cutomer_invoices:ORA-01400: cannot insert NULL into
("PERFIT"."OP_CUSTOMER_INVOICES"."FACTYP_FACTYP_ID")
Fixing this is quite simple, You'll go to Administration > Corporate Setup Manager > Repair Locations and select the location seen on the top left of the previous image, Invoicing Location Thame,
Open it up by pressing the repair shop code "SHOP" and go to the "Types" tab, you'll then want to add the type "RENTAL" and division "RENTAL"
If we go back to the rental manager and attempt to invoice again we should no longer have this issue,
We can see they were able to be invoiced without any error due to the fact we added the facility type as seen above.