Can I ask a postdocter candidate to write a recommendation letter for applying to graduate school 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.