Serapith

Hey, i just met a few days ago a little problem, where i can't complete the session, cause the skill i try to use is "unavaible". I made a screenshot, as u guys/girls can see, the skill's icon in grey, which means (i believe) it's not possible to accept it, even if i use the skill. I tried different timings, distances places from where to use it. I hope you can help me to solve this problem! https://prnt.sc/qgk2nm Thanks!