Is it okay to email professors/PI's to show interest working in their lab when you're applying to the school's Master's program? I saw on another question that unless it is explicitly said, you shouldn't contact the professor directly and should email the admission staff instead. There is a section on PI's webpage with his contact information, but in the Positions section, there are none available currently. Should I not try contacting this person at all?
In terms of the content of the message, I wanted to discuss my interest in a specific field and how working in his lab would help in achieving my goal. Furthermore I wanted to ask for any type of advice which may help when entering graduate school in the field.