Where are you trying to buy the game? I bought in on Steam using a credit card... there are also other methods in Steam you can try out if you're trying to buy in on itch.io then I can't answer you... I didn't buy it here.
Thanks for the advice! Unfortunately, I purchased the itch.io version, and it seems that the itch.io version is not compatible with the Steam version of the DLC. I will consider whether to switch to the Steam version.
