Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
You just ran something and the engine stopped dead, saying a builtin was not found in the execution law. Let's unpack that. The execution law is the engine's list of every builtin it knows how to run, each one declared in a file called builtins.feml. When your script calls a builtin, the engine looks the name up in that list. This time, the name wasn't there. Either it was never declared, or it was declared and then unplugged. Now, here is the important part. Normally this is not your typo. A misspelled name gets caught earlier, with a friendlier message. Getting this far means the parser or the lowerer produced a call to something that does not exist, or the builtin was declared but never wired to real code. Picture a phone book with a name crossed out: the number is gone, but someone still tried to dial it. The fix lives on the engine side. Check that the builtin is declared in builtins.feml, and that its wiring is in place. If you are just writing scripts, report it, because you have found a gap in the engine, not a flaw in your code. Remember: the message tells you which name failed. Start there.
Text from the Rush engine's error catalogue.