Why is "Ich ankommen" wrong?
Because you need to use the correct inflected (= finite) form of the verb for the 1st pers. sing. in Präsens (present tense), and that is (for this verb with a separable prefix) ich komme an.
ankommen is the infinitive.
An vs Ab?
Ankommen is a separable verb. Not sure why its in the preposition lesson, other than the "an." Is "an" still a preposition even when its part of a separable verb?
so this is 'I come at' ?
or 'i come about'?
i dont really get why you need the "an"
because the verb is "ankommen" (= "to arrive"). "kommen" is "to come".