Is salah valid if the first tahiyya/tashahhud is forgotten?

Salam I want to ask, what to do if a person forgot to read Tahiyatul during the second rak'ah?

Alaikumassalam wa ra rahmatullah,

The first tahiyya is sunna, not fard, so forgetting it does not nullify the prayer. If you forget it simply do sujud al-sahw after the prayer. Since sujud al-sahw is also considered sunna by some schools, forgetting to do this will not nullify the prayer either according to this opinion.