Skill allocation

[Onmyown23]

Hi all I have a message saying I have 1 skill not assigned but when I go to assign it I can only replace one of my existing skills even though it highlights the option to use l1+r1 together , how do I assign my third skill to these buttons , do I need to reach a specific level or upgrade the base to a certain level .

thanks in advance 

 

[TheDarkZone]

That spot is reserved for 1 signature skill. You have to upgrade a wing to 100% completion to unlock that wing's signature skill. The signature skill for each wing is the skill at the bottom of your skill list in each wing category.