What is this expression error?

[metalheadmortal]

Hi Im using expressions to drive blendshape and I keep getting these error. How do i fix it because im about to break my monitor

[osidedan]

I downloaded your scene file, and everything seems to work as expected! I even added a print test to your expression and I get no errors, and it prints as expected:

I'm on Maya 2018, but your expression seems legit to me.

My fix suggestion:
It may be a matter of Maya needing a restart.

I have have had to restart Maya many times before when working with expressions. Sometimes they just randomly stop working as you would expect, especially if you've been re-writing your expression multiple times.

I hope a restart helps, and I hope you haven't broken your monitor yet haha!

[metalheadmortal]

Im trying to use the blend with a different control I get an error when i type in 

L_BrowBlend.L_OuterBrow_Down = clamp(0, 1, -L_BrowControl.translateY); try this. Im pretty sure the spelling match up

[osidedan]

I got the error that time! thank you for adding that extra bit for clarification.
So I did a quick look at your setup, and it looks like "L_OuterBrow_Down" does not exist for your blendshape setup:

As a double check, I did an attribute query from the script editor, and it indeed does not exist. The Up one says it's there, but the down one does not. Maybe you need to re-add it to the blendshape? Here's the MEL for script editor checking:

attributeQuery -node "L_BrowBlend" -exists "L_OuterBrow_Up";
//Returns 1 

attributeQuery -node "L_BrowBlend" -exists "L_OuterBrow_Down";
//Returns 0 

So yea maybe just try re-adding the L_OuterBrow_Down to the blendshape, and that should fix it. Hope that helps!