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.

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neogodless a day ago | parent [-]

All I've never done the diagonals before... oops.

https://imgur.com/a/Dfxf9CJ (before picking 2 / 6 / 7.)

	▲	shagie a day ago \| parent \| next [-]
		https://www.sudokuwiki.org/sudokux.aspx is a useful site. I'm personally a fan of the killer variety. Its solver is interesting in showing the individual steps. For the same puzzle, https://www.sudokuwiki.org/sudokux.aspx?bd=06039000804900150...
	▲	zem a day ago \| parent \| prev [-]
		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.