A: So, what did you think of my singing?

B: Er, don’t give up the day job!

Note: If someone tells you not to “give up the day job,” they are saying that they think you should continue doing what you are good at, rather than trying something new which they think you will fail at. Usually it’s a humorous way of telling someone not to pursue something full-time as they are not very good at it. The origin of this idiom is unclear. For example: “If I were you, I wouldn’t give up the day job; you’re never going to be a professional dancer.”