What is the ruling on receiving a monthly salary from a "Saudization" company without working or attending, with the company's knowledge and consent?

If the company's employment of you without assigning work to you is not a circumvention of state regulations, and you are dedicating yourself to work, then there is no harm in the salary, as long as you enable the employer to benefit from your services, even if they do not fully utilize them. A condition for this to be permissible is that the company's contract with you without work is not merely a ruse on their part to evade the law. If it involves deception and trickery, then cooperation with them is forbidden.