That unhelpful error message might be an indication that the seller does not have shipping specified to your location or else needs to add your username to the seller's exemption list as you may not meet the seller's buyer requirements due to your location.
https://community.ebay.com/t5/Ask-a-Mentor/impossible-to-pay-for-an-item/qaq-p/26478658
In some cases, that error message appears along with another error lower on the page that says "the seller does not have that quantity available" or something similar, so apparently there are other situations where that message may apply.
