Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
You wrote a key in quotes inside brackets, and the engine pointed out an easier way. The rule is clean. A key is text, and it is never quoted. It qualifies the name directly, with a dot. So, for a header called content dash type, you simply write headers dot content dash type. The bracket form is reserved for values. A key held in a variable, a computed position, or a range. The old spelling, with quotes inside brackets, was used in the past, and the language narrowed it away. So the engine refuses it at parse, and shows you the replacement. Is there any case where quotes stay? Yes. If the key could only be produced by a computation, like a text made only of digits, it stays a bracketed value. After all, a bare seven after a dot means position seven. But a normal key, with letters, digits, dashes and underscores, is written bare. Drop the quotes, drop the brackets, and keep the dot. Shorter, and easier to read.
Text from the Rush engine's error catalogue.