"Kapan dia mengajarkan ini kepadamu?"

Translation: When did he teach you this?

[joK8Un]

as there is no tense marker why is when does he teach this to you wrong?

[LucasHains]

It's not incorrect - your translation is fine.
Actually, if you wanted to say "When did he teach you this" it should be "Kapan dia mengajarkanmu ini?" but it adds a degree of difficulty to the learner because you are meant to be learning about me-an words so me-anmu adding a mu suffix to this would confuse those who are not familiar maybe.

[nuyyy]

why i cant add about?

it should be correct if i answer, "when did he teach you about this?"