Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
You asked to turn a value into a path, and the engine said that value cannot be one. Here is the rule. Converting to a path only works for text, or for something that already is a path. The result is the very same text, now carrying the path kind. The engine does not check whether it looks like a real filesystem location, that is not its job. But a collection, a number, or a truth value is not text, so there is no path it could be. The engine will not turn them into text first and call that a path, because that would be pretending. Not everything is convertible to a path, and that is on purpose. Also notice that the empty text is refused too: an empty path is no path at all. Example: to path of the number forty two refuses, but to path of a quoted folder name works. The message shows the value you gave, and the reason, such as the collection's shape, the value's type, or the empty text. Start from real text, and the conversion will succeed.
Text from the Rush engine's error catalogue.