What should I do regarding the error in the supply and incorrect installation of ceramics, and the resulting losses, taking into account that the engineer responsible for supervision receives a percentage of purchases, and the company employee refuses to rectify the error?

You are permitted to claim your full right due to the damage inflicted upon you, even if it leads to the employee's dismissal; for the one who errs bears the consequence of their mistake. The majority of scholars hold the view that compensation for damages and the rights of individuals are never waived, and there is no difference between error and intent when it comes to people's property, because error falls under the category of "Khitab al-Wad" (enabling law) which necessitates guarantee from the one who errs, even if they were negligent or insane. And Allah has made obligatory the diyah (blood money) and kaffarah (expiation) for accidental killing.