I have some flights coming up on avianca that I booked using Aegean miles. Can't select seats on the Aegean nor Avianca websites - the Avianca website gives the same error as the OP to call them.

So, I called avianca and was told that their payment system was down, so would need to call back later or otherwise choose a seat at online checkin. Apparently if their payment system wasn't down, they would be able to charge me for a premium or exit row seat, but their system wouldn't let me choose a standard seat (neither for free nor for a fee) in advance.