Ik reken op jou. (I'm counting on you.)
rekenen takes "op" (rekenen op = to count on). Example: Ik reken op jou. (I'm counting on you.)
Ik reken op jou. — I'm counting on you. The preposition op is fixed, so it doesn't change with the sentence.
Fixed prepositions come up all over the A2 exam. The platform drills them in real exam tasks, AI-graded across all five DUO sections.