Basically I’m trying to block out eyelids on this character using the bend curve modifier. I know I could just do it on one side then mirror and weld, but I want to see if it’s possible to do both sides at once using symmetry. This message I get whenever I try to use the deformer suggest to me it is. 
So how do I “update the masking to include symmetrical parts?”
Thanks!
You’re probably well past this issue but I was watching a ZClassroom video and remembered your post and error message. The message appears because you have Symmetry active (Transform:Symmetry) when you are using a Deformer which hosts it’s own Symmetry settings. To work with the Deformer symmetry settings disable Transform:Symmetry (X key shortcut).
See Project Primitive Deformer from about 1:50.
Cheers.