Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
You asked concatenate to glue two things together, and they were not the same kind. Concatenate can join two texts into one text, or two lists into one list. The rule is that the second thing must match the first. Why so strict? Because gluing a text to a number would require a hidden conversion. Should the number become its digits? Should it be added somewhere? The engine would have to guess, inventing a conversion you never wrote. And a guessed conversion is exactly how quiet bugs are born. Same for a list beside a text, or a map, which has no natural join at all. So the engine refuses and shows both operands as you wrote them. Here is how to fix it. If you meant to build a sentence with a number in it, convert the number to text on purpose, then concatenate. If you meant to extend a list, make sure the second one is a list too. Say what you mean, and the join will work.
Text from the Rush engine's error catalogue.