Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
You just ran a script, and instead of a number you got an error that says: a text that is not a number. Let's talk about what rush is telling you. When you call to.number on a piece of text, rush tries to read it as an exact number. It accepts digits, at most one decimal point, an optional sign, and even an exponent. So one e three reads as exactly one thousand, no rounding, no surprises. But rush will not guess. If the text is the word garbage, or an empty string, or one point two point three with two dots, there is simply no number in there to hand back. Instead of quietly inventing a zero, rush stops and tells you which text it saw and why that text failed. Picture a script that reads a price from a form field, and the field says twelve dollars. That is words, not a number. The fix is to clean the text first, keeping only the digits, or to check the input before converting it. The same error also shows up with to.bool, which only understands the number zero or the number one. Not the text one, not two, not null. Read the two slots in the message: the first shows your value, the second tells you the reason. Fix the input, and the engine will happily go on.
Text from the Rush engine's error catalogue.