If the stock you are trying to short has a high chance of going down, it looks like they shut the stock as they don't want to lose money. That's what I gathered from some googling.
I'm a new user like you, and was trying to short Luckin Coffee on practice mode today. The chance of them having to pay me was high (practice money but whatever) so they didn't allow the trade.
However, since the stock has a low chance of rising, I can go long! Because I'm guaranteed to lose money and they have no problem with that. Quite shady if you ask me. I'm going back to my usual platform for CFDs - Luckin Coffee is open there.
