let (someone) down
Definition: to disappoint someone by not doing what they were relying on you to do.
E.g.1: Ted said he would drive me to the airport but he let me down at the last minute and I had to get a taxi.
E.g.2: We’re relying on you to do it. You won’t let us down, will you?
This phrasal verb can be separated (E.g. 1 & 2).