It might be better idea to show us how you use that (eg. the screenshot of the rule) but in general forcing it to process tag as a number with TN function might be needed.
It will be awesome if you can also let us know how you tried it I mean where you typed it? How did you configured your calculations? How you’re triggering it?
You can’t execute expressions in actions. You should type [:Discount Amount] to configure amount parameter as a variable.
You need to edit rule that you execute that action from. First remove that action from your rule, save it and re-add this action. When you expand the action box you’ll see Discount Amount added as an editor under action.
You’ll type your expression there but you also have a syntax mistake. You shouldn’t have curly brace in front of expression.
I have a product tag that determines what level of discount either 10% or 25% and changes the order state to SDRINK for 10% and SFOOD for 25% when these items are ordered.