remember, MC is considered a "bug", the moment she kills Yuriel in the far future, because the system considers her a "bug", MCs soul literally dies afterwards and since the protagonist Yuriel dies, the world does not know how to properly move forward. The plot becomes more complex later on but that's the basic answer of
Why she can't kill Yuriel
-Because she's a person from modern times who is actually a nice person who doesn't want to dirty her hands.
-MCs initial goal is ofc to go back home. If it means her death, it would be fine but it WOULD be better if both reincarnators had helped each other out. The only reason MC finally makes a move is because Yuriel denied her subtle offer to join hands as well as involving innocent people for her benefit.
-Because the system favors Yuriel, MC HAS to lead her to the bad ending instead (so basically, take advantage of her choices and find a way to make it lead to the bad end WHILE also winning over the capture targets)
-Again, the latest on the spoiler threads finally gave an answer, MC is a bug/irregularity of the world, if something were to happen to the heroine, the capture targets would lose trust towards her and lead to her doom, but since later on she already won all of them over, and gets the motivation to finally off her *Yuriel dies via a barrage of arrows aimed at her while escaping apparently*, MC dies moments later. You can read more spoilers in the spoiler threads on NU