Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
You wrote a call, and the engine pointed at a missing piece: a required parameter was not given. Here is how it works. When a builtin is declared, each of its parameters is either optional or required. Required means the builtin cannot do its job without it. The engine checks this at parse time, before running anything, so you find out immediately and nothing has been half done. The message names the parameter you left out, so you do not have to guess. Think of filling in a form where the name field is marked with a star. You can skip the phone number, but if you skip the starred field, the form bounces back. Say a builtin needs a file name and you called it with nothing at all. The engine cannot invent a file name, so it refuses and tells you which parameter is missing. The fix is simple: add the argument. Check the builtin's declaration if you are unsure which parameters are starred, because builtins.feml is the source of truth. One more thing: the order and the names matter, so make sure the value lands in the slot with the right name. Add the missing piece, run again, and you are through.
Text from the Rush engine's error catalogue.