Hi there!
This error means that RPG Maker is not finding an actor. As I can see on your event command, you are using `$gameActors.actor(0)`. Actor with ID 0 does not exist. The minimum valid ID number for an actor is 1.
If what are you trying to do is to increase the variable with the value of the custom parameter from your player/party leader, you can try do this:
$gameParty.leader().cparam(0)
or
$gameParty.members()[0].cparam(0) → Here the 0 will work, because we are not poiting to an ID, but for a position(INDEX). And indexes, always start at 0 on javascript.