Is it permissible to place a marker on a grave to distinguish it and facilitate its visitation without violating the Sunnah, knowing that there is a saying: "May Allah have mercy on a grave that is not known"?

There is no harm in placing a stone or similar object on a grave as a marker to distinguish it, and this is the opinion of the majority of scholars. The Prophet, peace and blessings be upon him, did this with the grave of Uthman ibn Maz'un so that it might be known, and he said: "I mark with it the grave of my brother, and I will bury next to it whoever dies from my family." As for the saying "May Allah have mercy on a grave that is not known," it has not been found and its attribution to the Prophet, peace and blessings be upon him, is not believed to be authentic.