Complete the frustrated chef's text message to his best friend.

I spent six hours sculpting a life-sized swan out of butter for the gala, but since the event was abruptly moved online, I ______________ all that effort!

The correct answer is needn't have bothered with.

The structure needn't have + past participle is used to express that an action was performed in the past, but it was completely unnecessary and a waste of time.

"Mustn't have bothered" is a negative deduction meaning "I deduce that I didn't do it," which contradicts the fact that the chef spent six hours on it. "Couldn't have bothered" expresses impossibility, which also does not fit the context.