I'm trying to create a snippet that includes a bit that codes for rolling 1d8 + 1d8*your highest spell level for a warlock, which caps at 5 (basically making a subclass ability that is similar to Eldritch Smite.

I would like that snippet to, ideally, work like Hex Weapon and add it to the roll.  But that doesn't seem to be an option, so I'd like to have a bit in the snippet that lets you roll the extra damage there.

I am using Unable to parse dice roll. but that doesn't seem to work - I assume it's the syntax of it?  

Can anyone suggest a way to fix it?

Snippets don’t actually add anything to rolls. Snippets just display things. Likely they added an actual fixed value with level overrides or something, and then just used the {{fixedvalue}} snippet. Or perhaps the entered a scale value and used the {{scalevalue}} snippet. The easiest way to check is by making a homebrewed warlock subclass using Hexblade as the template and dissect that to see how they did it.

I am literally making a Hexblade variant and did not think of doing that.

Thanks!