I am from Greece.I called our Samsung service Center and they told me that if the knox is triggered then they have to replace the motherboard of the phone in order to repair anything.So the motherboard costs about 300€ .This is a joke 6 days old phone and have problem with the screen without even dropping it
If the implication here is that it's not possible to repair it without replacing the motherboard, then that is complete bulls**t and little more than a scam to get more money out of you you. There is absolutely no reason a screen can't be replaced on a phone with a tripped knox flag. Even if they refuse to do it under warranty thanks to the tripped flag, the most you should be charged is the labour to replace the screen and the screen itself.
If this is a new Samsung policy that no knox tripped device can be repaired without replacing the system board first (even if the fault is in no way related to the system board) then it's a new low from them.
But unfortunately, in terms of not getting it repaired for free, this is the chance you take with tripping your knox and you will have known that prior to doing it. Take it to a third party repairer would be my advice. It would be far cheaper, although you would risk no longer having your IP68 rating.