Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
You used pick or exclude on something that has no members to choose from. Let's see what they work on. Pick and exclude choose members by key, so they need a map. They also accept a list of maps, and then they work row by row, applying the same choice to each map in the list. But a single scalar, such as a number or a text, has no members at all. And a list that contains something other than maps, say a number among the rows, has a row with nothing to choose from. Rush refuses and does not try to make something up, like returning the value untouched. The first slot names the operand, and the second says what was wrong, and for a list it gives you the row number, so you can find the exact member that does not belong. Imagine a list of ten users where the seventh row is a stray text. The message points to row seven. Fix that row, or remove it, and the choice will run on every map.
Text from the Rush engine's error catalogue.