When you want to transfer USTD, type of the blockchain is important.
Because gas fees are paid in native currency of chain
For example if you are using USDT on ETH chain gas fees will be covered from native currency of this chain witch is Ether
Pleas make sure that you enough amount of Ether to gas fees of this transaction
Also you can report problem directly to Opera Crypto Team via wallet. We will see your report much quicker. In wallet settings you can click on "Support" button on the bottom section of wallet settings.
Opera Crypto QA Team