You don’t need the [exists] conditional in there, you’re using an out dated macro format from ~2008-2009. The [dead] conditional has the [exists] conditional baked into it now.
This should work just fine.
If you can skin the target, it will skin, otherwise it won’t.
/targetlastenemy
/cast [dead] Skinning
This would be an easier (imo) macro to use, just hover your mouse over and it’ll skin whatever it sees until there’s nothing left.