I bought Scoundrel’s Missing Boots from the transmog vendor next to the stormwind trading post when the trading post first came out. When I try to learn the appearance I get a “You cant’ do that right now” error. This error also occurred during the Trial of Style, which I thought might have been the issue. (I’m only just now remembering to submit a ticket)
I have the same issue, Hunter also if that matters.
Even though it’s a cosmetic item, it’s still tagged with an armor type (leather) which means the usual class transmog rules apply like it was a real piece of armor: hunters can’t learn a leather transmog, because their “preferred”* armor type is mail. Just because you could put on a piece of leather armor doesn’t mean you could transmog it. You can only transmog items of your class’ specific preferred type.
*“Preferred” really means “required”. If you wear an item of armor of a type other than your class’ type, you are hugely penalized. And you can’t learn the transmog.