I bought 3 days ago MIM HSS strat, i have fallen in love with it except that damn low E string that keeps buzzing only on open.. .when i fret it nothing happens... somehow buzz seems to go away if i press the string on top of nut ( a bit , sustain is same ), so wtf is wrong ? When i got guitar it was tuned in D# maybe truss rod needs to be adjusted?

Sounds like the low E's nut slot is either too low, or not angled enough. If the truss was straightening the neck too much, you'd get the buzz. But it would more than likely happen to the treble strings first, as they are cut a tad lower than the bass strings.

Or as a slim chance. You have restrung the guitar, put too many winds round the tuners and they have crept up the tuner post on the low E. Reducing the break angle over the nut.
If that's the case, take the string off the tuner. Shorten it so it only pulls to around 40mm past it's tuner post. Then wind it back on, making sure every wind goes below the last.

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No no and no

I don't know how to explain but all strings when looking from a profile at the nut seem to be 'fit' into nut , except 6th string which is like a hill on the nut, at first look i thought it's gonna fall off... it looks like the hole is not wide enough for string to get into it...i just wanna what it is , so i don't like an idiot when i give this guitar to a store where i bought it

it's possible that the nut is broken at the 6th string slot. that's what happened to my telecaster. i super-glued it back together about 4 years ago... still working nicely.

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No that's right. Only about 40% of the low E string should be in the nut. That shows the top of the nut has been cut correctly.
There should be between 60-80% of the A string in the nut. The rest should be fully in there.

Though if the slot bottom and sides are cut well, it doesn't really matter. It's just to stop the top of the slot interfering with the strings vibration. And to make sure that the bass end of the nut doesnt become uncomfortable if your thumb should slide over the top of it towards the headstock

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No no and no