What does the message "No opening trades: ineligible PINK stock" mean?
When you see the message "No opening trades: ineligible PINK stock" it means an opening order for that contract is not allowed, even if your account has trading permissions for US Penny Stocks.
Yes, you can short Bitcoin futures. Find more information on margin requirements on short Bitcoin Futures positions in the Trading Bitcoin Futures with IBKR article of the IBKR Knowledge Base.