How do I unlock shadow strike skill?

[Doomguy]

When I try to unlock it ,it says parent skill not purchased
But I can't find parent skill !
Any help please?

[Blackwolfe]

You did purchase the previous skill in the line right? And also I assume you are the correct level to unlock shadow strike?

[Crackjack]

Have you unlocked the base skill?
You know there is the base skill/parent skill
and then there is the perks which are upgrades for the parent skill