Samsung Galaxy S6 — S6 (SM-G920F) can't downgrade from Android 6.0.1 to 5.1.1...?!

Hi everybody, after googling a lot, I now come here, because I'm desperate .. The thing is: I would like to downgrade my S6 on Lollipop, because at this time you csnt do many things with root with Marshmallow ... The only problem is if I go through the known procedure with Odin, so stock firmware flash, etc. (and indeed I have the Samsung USB Driver installed). However, he tells me in Odin top left, almost immediately after the start of the flash: FAIL! (In red) ... Have no idea what to do...

Samsung Galaxy S6 — S6 (SM-G920F) can't downgrade from Android 6.0.1 to 5.1.1...?!

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