What is the ruling on money earned from working as a marketer for training courses, where a specific commission (10-15 Riyals) is received for each person who registers through her, without receiving a fixed salary or paying money to the center?

The mentioned transaction is brokerage, and brokerage is mediation between the two parties of a contract or guiding to something. There is no objection to it, nor to the earnings gained from it, as long as it does not involve aiding in something unlawful. Whoever performs the work is entitled to the fee as a fixed debt, because the contract is established upon the completion of the work.