why this bluff does not make sense ?

To understand the validity of your bluffs you have to follow your perceived range (the cards you are representing):

In this example, you call the flop which makes your hand look like 9x, or more likely a PP.

If you had the 9, on the river you would either c/r or bet big, you wouldn’t bet small. As you did bet small, he can now take 9x from ur perceived range. Leaving only weaker PPs in your perceived range.

You showed up with KQ which the villain wouldn’t have assigned to your range. He wouldn’t have assigned this hand because calling oop is not something you want to do with absolutely no hand! In a mwp, this is basically something you never want to do.

Bet too small to scare him off any hand, go bigger if your bluffing, it’s not like he checked to you first and you bet half pot, your OOP you have to go bigger. Another thing make sure your opponents player type is bluffable

You can sometimes go small killa’, what’s important is that your story makes sense:

If you’re repping a mediocre hand, betting small as a bluff is often more effective than betting big. If you make it look like your going for thin value he’ll often fold those weak value hands.

Sometimes you can’t bet big:

If you have no nutsy hands in your perceived range but you have many bluffs + mediocre hands, and you then try to bluff big, nobody will believe you and you will lose an expensive pot.

Bluffing small is better because it’s cheaper, so don’t mess these ones up by betting big! Follow your perceived range and you’ll never make such a mistake.

Having said all that, I do obviously agree that in this OP it’s better to bet bigger!

We should fold the flop to the preflop aggressor who is cbetting in to 3 players.We have 2 players left to act after us and all we have are over cards with some backdoor straight equity.Maybe i could get on board with floating the flop but the turn call is pretty bad imo.

This is NL2 and than bluffing isn't a great plan in the first place.
Especially when you start the hand multi way post-flop.
Flop would be already a fold multi way with 2 over cards and no need to make fancy plays on NL2 I think.

Cheers,
SDK1987