Is saying " I seen it " grammatically correct?

It ain't.

I've seen it = present perfect
I saw it = simple past

Yes - "I saw it" or "I have seen it", occasionally "I will have seen it"; depending on context and sense.

I forget the grammatical theory but the choice correct at the time, sounds or reads correctly, therefore sensible and mellifluous.

No and yes

I've seen it.

No. Yes

In a conversational format you can make as many grammatical errors as you like "Because people aint all talk the same, and truth is, some don't care to".

I see it
I saw it
I have seen it.
I have been seeing it. 😉

I see it, I saw it, I have seen it

It’s I have seen it

I seen it both ways 🤷‍♂️

I prefer "I seen't it"

I've seen it, is the correct way to say that

No it's I has seens it

I looked at it.