Learn Hardcore Russian: Я пока не уверен, смогу ли я прийти вечером. - I’m still not sure whether I’ll be able to come in the evening.

So Я пока не уверен is literally something like I am not sure for now, but in natural English it is usually I’m not sure yet or I’m not sure at the moment.

This is a very common use of пока with temporary situations:

Я пока дома. = I’m at home for now.
Мы пока не знаем. = We don’t know yet.

It does not mean while in this sentence.

Why is it не уверен, not не уверенный?

Because after Я you need the short-form adjective: уверен.

Russian often uses short-form adjectives after verbs like to be (even when быть is omitted in the present tense):

Я уверен. = I am sure.
Она уверена. = She is sure.
Они готовы. = They are ready.

The full form уверенный usually describes a noun:

уверенный человек = a confident person
уверенный голос = a confident voice

So:

Я не уверен = correct
Я не уверенный = unnatural in this meaning