This is an idiom that means Solomon does not know how to govern as king. AT: "I do not know how to be the king" or "I do not know the right way to do things as king" (See: [[rc://en/ta/man/translate/figs-idiom]])
Solomon asks a question to emphasize that he knows that no one can judge. AT: "No one is able to judge this great people of yours." (See: [[rc://en/ta/man/translate/figs-rquestion]])