…and I have reported it directly to support but figured I’d see if anybody else here had the issue and at least you’d know someone reported it 
I cannot purchase a character slot, I get the popup prompt (which also lacks the Amazon Coins option like other Fire TV apps have and I reported that too). I wanted to get another character going to hire, and was bummed that I couldn’t make the purchase. I selected the 0.99 option, hit the right buttons and… nothing.
So I did find a workaround, though. I downloaded DQ on my iPhone and logged into my account. I downloaded my existing character and then purchased a second slot and created a character there. I didn’t play at all with either because I don’t think I want to after using the controller on Fire TV which is definitely an awesome way to play. Anyway, I then uploaded back to my DQ account from the iPhone version. I then logged back into the Fire TV version and downloaded my account and it worked.
I’m kind of bummed because I had Amazon coins but luckily it is only 0.99 anyway. I hope we can make purchases (honestly I didn’t try any others, so maybe they work -but still no Amazon Coins option) with Amazon Coins in the future.
Overall this is REALLY nice on Fire TV and I applaud SB for bringing it there and getting me hooked!