a chicken-and-egg problem

a chicken and egg situation/problem

A situation in which it is unknown what happened, or what needs to happen, first. You need experience to get a job, but you need a job to get experience. It's a chicken and egg situation.

a chicken-and-egg problem

an unresolved question as to which of two things caused the other.
This expression comes from the traditional riddle: ‘which came first, the chicken or the egg?’
See also: problem