To "let down" someone in that context is to disappoint or fail to support that person. Perhaps the person hoped for or expected an outcome and ended up feeling "let down." "I'm not the kind of girl that you can let down." It's a statement of warning in a way. If you "let down" that girl, you will regret it in some way. For example, the termination of a relationship.

Okay, I just got the "whole sentence." I think that the gist is the same. It won't be "okay" if she feels "let down." That's my two cents!

sorry I need to write down the whole sentence: I'm not the kind of girl that you can let down and think that everything's okay Boy.

Does it mean " I'm not a girl who is easy to get disappointed by you"?