The most likely reason for a stiff link is if it's just been connected.
Diagnose the stiff link
If the link has just been worked on you know where it is, otherwise check for stiff links on the bike by turning the pedals backwards and watching for a twitch in the rear gear mech. The thinness of chains designed for 9, 10 or 11 speed systems make it tricky to join them with a tool, they are supplied with trick links. You do not need to remove the chain from the bike to spread a stiff link.
Identify The High Side
Once you have identified the stiff link decide which end of the pin is standing highest from the plate. You can do this by sight or touch.
Place the chain on the teeth nearest to the punch with the high end of the pin facing the punch. Screw the punch in until it’s resting against the head of the pin.
Check the link is free. If not repeat the last three steps.