For one of my class options, I want to give the player Hellish Rebuke, with an amount of uses equal to their proficiency modifier. I can set it to a fixed value, but using {{proficiency}} gives the error "{{proficiency}} is not a valid number". Is there a possible workaround?

You could have it set to scale at different class levels equivalent to proficiency bonus increases. This also prevents multiclass exploits so to get more uses they must invest more in the class.

Since this is not a bug and something you can, in a way, achieve with the homebrew tools I think this should be a Homebrew section thing. There people can give you better advice on how to achieve what you're asking. As a tip: snippet codes ( {{text}} ) are not variables: they only work in the Snippet box and only to serve as a way to make info more concise for displaying on the character sheet.

You can't set spell uses to scale with class levels, only actions, I think