Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
You used each with data dot add, and the call was not shaped the way each requires. Let's see what each does. Data dot add with each takes a collection, a list or a map, and adds every member of it as its own member of the holder, all in one statement. It is like pouring a whole box of items onto a shelf, one by one. For that to be clear, each must stand alone in the call. If you write it beside another value or another label, one statement would be doing two jobs: spreading a collection, and adding single members. Rush will not guess how you wanted those to combine, so it refuses. And the value after each must really be a collection. A single number or text has no members to spread. The first slot names the holder, and the second says what is wrong. If you want both effects, split them into two calls: one with each, and one with the single value. Each call then says exactly one thing.
Text from the Rush engine's error catalogue.