used to tell someone that you think they are not really good at something and they shouldn’t invest in it.
You obviously like playing the piano and seem to be enjoying it but I think you shouldn’t really give up the day job, Mary.
don't give up the day job (informal ,humorous)
used to tell sb that they should continue doing what they are used to, rather than trying sth new which they are likely to fail at
ٍExample: So you want to be a writer? Well my advice is, don't give up the day job.
Oxford Advanced Learner’s Dictionary, 8th edition
© Oxford University Press, 2010
don't give up the day job! humorous
used for telling someone that you do not think they are very good at something:
'What did you think of my singing, then?' 'Er, don't give up the day job!'
Cambridge Advanced Learner's Dictionary - 4th Edition
© Cambridge University Press 2013