Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
You asked rush to pick or exclude members of a map, but the keys you gave are not keys. Here is the idea. Pick keeps only the members whose keys you name. Exclude does the opposite: it drops those members and keeps the rest. In both cases, the keys come from the keys argument, and a key is always a text. You may give one text, or a list of texts. What is refused? A map instead of a list of keys. A list that contains a collection, or a null. Or a null by itself, which names nothing. Rush will not try to guess which members you meant, because choosing the wrong members would silently change your data. The first slot shows the operand as it was seen, and the second shows the lawful spellings. Say you have a user map with a name, an email, and a password, and you want to share it without the password. Exclude with the key password. Give plain texts, and the choice is clear.
Text from the Rush engine's error catalogue.