Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
Your script uses a keyword that rush has retired, and the parser stopped you at the door. Here is the story. Some words used to mean something, and then a ruling took them away. That is different from a typo, where the word never existed. Rush remembers retired words, and when it sees one, it names the ruling that retired it and tells you what to write instead. Today's example is parallel. In rush, every set of siblings in the script already runs in parallel, loop passes included, with no exceptions. So writing parallel for, or a parallel block, marks nothing that is not already true. It would be like writing wet on a glass of water. The word is redundant, so the engine retired it. The fix is the simplest one there is: delete the word. Keep the for loop, or the block, exactly as it is, and the engine will fan it out on its own. The message shows you the keyword, the ruling, and the replacement. Remove the old word, run again, and you are done.
Text from the Rush engine's error catalogue.