If you're referring to the yellow triangle on the "Buy" button, it's warning you that, as a subscriber, you can't directly *use* it because subscriber accounts have the unlock applied because of their subscriber status. (However, if you unsubscribe, the status-based unlocks go away, but you can get them back by using the relevant unlock.)

If the "Buy" button just plain doesn't work, please describe exactly what you see.

Well, please describe what you see anyway, so that people can help you better.

I went onto my least-provisioned pref account and saw this https://i.imgur.com/ZlVd63C.png

Only reason I couldn't buy it was for a lack of cartel coins.

Is this a bug?

Not exactly, the item you can see on CM is just per character, account option/item is missing.

With such price per character - I suppose account one will cost more than 3 month subscription.