to let (someone) down (to disappoint [someone])

a letdown (a disappointment)

Eg 1:

A: I made so many mistakes in the speech yesterday. I feel like I really let everyone down.

B: You didn’t let anybody down. We thought you did a good job.

Eg 2:

A: How was the picnic on the beach?

B: It was a bit of a letdown. Hardly anyone came, and it was very windy, so we had to eat in the car.