Trying to buy Nightmare Potion via the Token Shop, I get the error message "Oh no! You can't buy that here!" Why? I have enough tokens...

probably because there aren't any left.

It still showed during multiple refreshes. Also, why wouldn't it have the standard 'sold out' error?

hm I'm not sure if it was just being slow or not. I'm not sure what the sold out error looks like in the token shop, because really that could be the sold out message.

"Oh No! You can't buy that here!"

just tried to buy the moss covered rock (the link to the buying page was still in my history) and that's the error I got.