What should I do regarding the non-submission of my sick leave, which I obtained six years ago, after the former head of the department refused to receive it out of pity, and the head of the department has now been changed?

If the head of the department who has the authority permits you to take the salary, then there is no blame upon you. However, if he is not authorized, then you are only entitled to what is taken by someone who submitted a similar sick leave. The excess amount must be returned to the employer, or spent on the welfare of Muslims, the poor, and the needy.