Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
A library file in your project tried to print something while the project was only loading, and the engine stopped it. Here is the background. Library files, the ones that are not the entry, store templates. They never execute. There is one exception: their value binds. Those are laid down before the entry runs, so that another file can read a value like a dot v. But what if the right side of a value bind is a builtin that reaches the terminal, like write, or tui? Then merely loading the project would print on the screen. A project that is only being loaded should be quiet. So the engine refuses at parse. It does not silently run the print, and it does not silently drop it either, because both would surprise you. The message names the line and the builtin. The fix has two good options. Move the printing into the entry file, where execution is expected. Or turn the bind into a function that is called when needed. Libraries load quietly, and the entry does the talking.
Text from the Rush engine's error catalogue.