Because there's already another 6 on the same main diagonal. This is an "X Sudoku" puzzle, which means that each main diagonal must have all 9 different numbers.
replyAll I've never done the diagonals before... oops.
replyhttps://imgur.com/a/Dfxf9CJ (before picking 2 / 6 / 7.)
https://www.sudokuwiki.org/sudokux.aspx is a useful site. I'm personally a fan of the killer variety.
replyIts solver is interesting in showing the individual steps.
For the same puzzle, https://www.sudokuwiki.org/sudokux.aspx?bd=06039000804900150...
if you like sudoku, you owe it to yourself to check out the "cracking the cryptic" youtube channel and get introduced to the delightful world of variant sudoku.
replyYeah, that Sudoku puzzle has multiple valid solutions, whereas the page only seems to accept a single one.
replyThere's an extra constraint on that one: the two main diagonals must both have nine different digits. That's what makes it a single-solution puzzle.
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