How to use this variable [$Dmg] - help request

I use this variable often:  [$Dmg] within !powercards like this:   --Damage:|[[ [$Dmg] ((?{Spell Slot Level|3}+5)d6cf<0cs>21) ]] ``fire`` damage! When I call it back it becomes [^Dmg]. But when I try to roll a dice and include it such as [[0d0+ [^Dmg] ]] I get 0.  Any suggestions? Thank you!

That syntax is part of the powercards script, and doesn't work for anything else. There are ways to use modifiers from a character in standard roll20 macros (which look like @{attack} or @{selected|attack}), and ask for numbers (like  [[1d20+?{what bonus?|0}]] ). Check out the wiki for more details.

Questions about the PowerCards mod (API) should go to the Mods (API) forum. However, IIRC you can't use a Roll ID like that in PowerCards. Use ScriptCards instead.  Roll IDs (results of rolls assigned to names that can be used in output as placeholders for the roll value) are NOT variables. All rolls are processed before the output portion of any PowerCard tag takes place, and Roll IDs from one roll are not available in other rolls. Roll IDs are only used to include the result of a roll in output or to compare the result in a conditional tag.