If a person has a concave forehead such that they cannot touch the turba with even the size of a thumb’s tip…

Answer

In the name of Allah, they should prostrate on one of the two sides of the forehead. If they cannot, then on the nose. If that is also not possible, they suffice with bowing to the degree of prostration. The obligatory precaution is to combine this with prostration on the chin, even if it requires repeating the prayer. If prostration on the forehead becomes impossible, then prostration on the chin alone is sufficient. Allah knows best.