unable to create an expression

You cannot loop compound properties like path keyframes. You have to pre-compose, enable time-remapping and loop that.

Mylenium 

What property are you trying to loop, and how many keyframes do you have? Vector Paths and layers with less than 2 keyframes cannot be looped. I don't see any layer properties in your screenshots. We need more details to figure out what is going on. 

Try the following:

loopOut()

More info here:

https://www.schoolofmotion.com/blog/loop-expression-after-effects