English equivalent of Russian "понаберут по объявлению" [duplicate]
Merriam-Webster: "out of one's league," and Collins' English Dictionary (for English Language Learners) : "out of one's depth" would both meet your needs.
Such a person is in over their head:
be/get in over your head: to be or become involved in a situation in which you do not have the necessary skills, knowledge, or money to succeed