Should one who is in a state of janabah and capable of performing ablution but incapable of performing ghusl perform ablution for prayers, or should one also perform tayammum for prayers?

If you are in a state of janabah (definition) (major ritual impurity) and are unable to use water for fear of illness, worsening an existing illness, or delaying recovery, then you must perform tayammum (definition). Wudu (minor ablution) is not sufficient as a substitute for ghusl (definition) (ritual bath). Tayammum does not remove the impurity (hadath (definition)); rather, it permits prayer when water is unavailable. Therefore, you should perform tayammum only when you intend to pray.