When entering a payment for a membership on a constituent's record and the membership amount is $0 you may notice that Altru will show this error message: select a valid payment method even though there is no payment for a $0 transaction.
Change the payment method to cash or check and complete the payment. Since the payment is $0 this won't throw off any of your revenue in Altru
Steps to Duplicate
Click constituent > constituent search
Select the constituent from the constituent search screen
Click add a membership on the constituent's record
Add or renew a membership and change the amount of the membership to $0