I'm working in a lab as an undergraduate worker. Can I ask one of my lab member who is a postdocter candidate for a recommendation letter?
Yes. Especially, a postdoc that has worked closely with you, and may have a different perspective than your professor. Whereas the professor may be involved in broader planning and thinking, the postdoc may have spent many hours with the student in the lab, analysing data, etc. Therefore, letters from professor and postdoc could complement each other and work very well.
You can absolutely ask a postdoc (or even a PhD student) for a letter of recommendation! It happens pretty frequently :)
If you're debating between asking the postdoc or the professor (PI) in charge of the lab, it depends on your interactions with each of them and what you think they might have to say about you. A letter from the professor may hold more weight due to their reputation--if they can actually say something meaningful about you. If you think they would just give you an obviously impersonal form letter, it might be more valuable to get one from the postdoc who actually worked with you and can provide a more detailed and personal recommendation.
If you're applying for a position in the same field as the postdoc, also consider whether that postdoc is already making a name for him/herself, or if the professor is new and hasn't developed much of a reputation... that might further weight the value of each of their potential letters.
I'd suggest asking the postdoc who they think would provide a more valuable letter, as they will know your research field and professor much better.