Can we use Pindrop instead of Pindrop silence in a sentence?

The director of our English training institute told us that we must use pindrop instead of pindrop silence. I have searched on the internet to find out the correct way of using this word but I could only find the usage of pindrop silence.

Am I missing something here?

I'm not familiar with "pindrop" as a single word, but there is a dictionary entry for the phrase "pindrop silence," including having "pindrop" as a single word. It is tagged as "Indian English." What I'm more used to, in UK and US English, is the more wordy idiom "[it was so quiet] you could have heard a pin drop." (Note, "pin drop" in this case is two words.)

Perhaps your teacher was directing you away from a usage that wouldn't be local to your context (unless you're specifically learning Indian usages). But I'm not aware of a usage that would use "pindrop," on its own, as an adjective or adverb (though, as noted in the comments, the hyphenated adjectival "pin-drop" could be readily understood).