Russia sets the bar for Turkey to join SCO: quit NATO.
Putin's special envoy to SCO explained it as that candidate countries cannot be in conflict with any current SCO member, cannot have ongoing external conflicts either, not sanctioned by UN, and not a member of international groups that are hostile to any current SCO member. He specifically named NATO as such international group which has declared Russia as its number one enemy.
But by these conditions, how were India and Pakistan allowed to join then?
My guess is that the SCO rules are and always have been in flux. Lots of exceptions can be made when the strategic calculus is overwhelmingly in favor. Turkey's case is not one of those. India and Pakistan are both nuclear powers with huge populations. Pakistan has close ties to China and India has close ties to Russia. I imagine that is the reason.