Is earning from internet sites permissible or forbidden, knowing that they do not advertise alcohol, but sometimes display advertisements for gambling sites?

There is no harm in taking a fee for advertising permissible companies or promoting a lawful matter. However, advertising a forbidden matter, and taking a fee for it, is forbidden. This falls under cooperating in sin and aggression, as Allah Almighty says: "And cooperate in righteousness and piety, but do not cooperate in sin and aggression." If some advertisements are forbidden and some are permissible, then the fee for the permissible is lawful, and the fee for the forbidden is unlawful.