How do I make turrets not shoot if there is collision between self and enemy?

Well, the title is very self explanatory.

Thanks in advance!

Turret doesn't shoot by itself, it triggers an event "On Turret shoot". Inside this event you can check that the turret is not overlapping an enemy, and only then create a bullet.

I think you didn't understand what I want, I want the turret to aim for a target only if there is no solid objects between them.

Your title says nothing about solid objects. "Collision" is when two objects touch!

You should use Line of Sight behavior. And instead of adding object to turret's targets, acquire and unacquire targets.

Here is a similar demo, you can add a solid wall to it and turrets won't shoot at targets that are behind the wall:

howtoconstructdemos.com/how-to-limit-the-angle-of-view-for-turret-behavior