Some characters can't speak five sentences without yelling about something. If these characters were to carry out an undercover mission, their habit of yelling will most likely make the mission more difficult — or foil it entirely. No one will want to carry out a covert operation with such loudmouths on their squad.
The first words Asta ever spoke were in an aggravating loud voice. He's one of anime's noisiest characters. In fact, a lot of viewers actually stopped watching Black Clover because of his constant yelling. Asta's yelling gets on everyone's nerves. It's amazing that he's never had a headache because of his loud voice.
Having Asta go for an undercover has to be the worst mistake anyone can make if they were hoping the mission will be successful. It's almost impossible to get Asta to be quiet. Perhaps if they had some silence magic, it'd be possible.