Did you check the Term setup in Accounting Lists with an internal ID of 15. Does everything look okay there? E.g. it’s not inactive or something else funky going on there. Is that a valid Term for all the customer you are invoicing? E.g. do you have an customer with a credit hold or credit limit or something else that would prevent them from being invoiced with these terms? Or maybe a parent customer has similar issues? Or maybe you’ve set it to only bill at the top level parent and so it doesn’t like you billing a sub-customer?

Basically invalid reference key just means “there’s some sort of issue/error encountered when we tried to plug in this value” so you’ve got to figure out what record they’re talking about (in this case the term with an internal ID of 15) and where plugging in that value would cause issues (which could honestly be a lot of places, but in this case is most likely related to the term itself or the customer).  Wish I could give you a more definitive answer, but when I’ve gotten these similar errors in csv imports and the like it’s always a bit of wild goose chase, but there has always been some sort of reasonable explanation for why that process should not be allowed to happen, even if the error message wasn’t very helpful in actually diagnosing it.