Rental charge codes should be set under a product class for rentals and as a non-inventory item. (By default, the product class for rentals is 4 – Equipment Rental. If the product class for rentals is different in your system you will need to set it to that class.)
Cause
RMA requires a truck be assigned to all invoice that have inventory. This is because inventory tracking requires a truck to be assigned to track movement of items between trucks and the warehouse. If a rental charge code is not setup correctly as a non-inventory item RMA will want a truck assigned to that invoice.
Resolution
You will need to do is go into Lists > Products > Product Charge Codes, go to the rental charge code that was used on the invoice.
Next, mark the product as "Non-Inventory Item".
Save this, do this for all rental charge codes or any product assigned under a rental class code. This will keep this posting issue from happening in the future.
Please note you will still need to manually assign a truck to the invoices this one time in order to allow you to post these invoices.