Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
You called char.at, a builtin that no longer exists, and rush told you so by name. Here is the story. Char.at used to fetch one character from a text. Then a ruling said: text selection is naming, just like lists. If you can select the second item of a list by writing its name, a dot, and a number, you should select a character of a text the same way. So the builtin retired. Instead of saying not found, as if you had made a typo, rush says this word was retired, and it shows what to write. For the first character of a text t, write t dot one. For the last one, write t dot backwards dot one. For a stretch of characters, write t dot, then the range in square brackets, two dot dot four. One idea for lists, texts, everything. And if you wrote your own function named char.at, yours wins, because a user function in scope always takes precedence. Replace the call with the selection form, and you are done.
Text from the Rush engine's error catalogue.