There's a repair option in the modules screen on the right panel. The repair button has always been red and uninteractable.
If there's some specific materials needed, if so, what are they? If not, how is the repair button used?
You need a Auto Field Maintenance Unit (AMFU) to use that. But if your FSD is at 0 and you are still alive, you can use Reboot-Repair in the last pane of your Systems panel, at the bottom; to revive it to 1% and limp home.
You have fit an afmu, (without it there would be no repair option on the modules screen), that requires special ammunition to use the unit, you can buy that on the advanced maintenance screen on stations. Note the ammo will take up cargo space, and the repair is a little on the slow side so is not viable in combat.