Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
You asked rush to append text to a file, and the operating system said no. When the target of an append is a file, rush adds the value's text at the end of it. That sounds simple, but the file system has its own rules. The folder might not exist. You might lack permission to write there. Or the disk might be full. Whichever it is, the operating system knows, and the engine passes the truth along. The message shows the real path you tried to write, and the real reason the operating system gave. The engine will never pretend that the write worked. A silent success here would be the worst outcome, because your script would continue as if the log line or the saved result were safe on disk, when in fact it never arrived. So what do you do? Read the reason. If the directory is missing, create it. If it is permissions, fix them or choose another location. If the disk is full, free some space. Then run again, and the append goes through.
Text from the Rush engine's error catalogue.