Learn English – What’s the difference between “turn out” and “end up”

I have two guesses. My first guess is that you have probably misheard the colleague. The correct form is "you know".

My Question: Why are you not taking any leaves nowadays?
My colleague's reply: I am in my notice period, you know.

From The Cambridge Dictionary:

We use you know very commonly in speaking. It is used to check with our listeners that we share the same knowledge. When we use you know, we assume that the listener or listeners do have the knowledge that we want them to have:

Well, what can I say, you know, I want my career to go far.

A: How was the match?
B: Well, they played OK but the defence was, you know, the same as always.

You know is also sometimes used to allow us time to think when we are speaking:

A: Did you like the play?

B: Well, I’m not sure, it was, you know, it was interesting.

The dictionary has a nice page on discourse markers.

My second guess is that if it's not you know, then it might be now. If it's just know, it seems to make no sense in that position.

With now, the dialogue looks like this:

My Question: Why are you not taking any leaves nowadays.
My colleague's reply: I am in my notice period now.

We react to a stimulus or situation.

The dog reacted to the bell by salivating.

The preposition on would not head a phrase which refers to that which is being reacted to; it might be adverbial, for example:

The emergency team can react on a moment's notice.

The dog did not react on the first experiment.

Or on could refer to something which is affected:

These chemicals react on plastics.

From a grammatical perspective, the plastics are not the stimulus. There is a reaction when the chemicals are applied to plastics.

We could say:

These chemicals act on plastics.

But people often say that the one thing reacts on the other.