Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
You used process dot run, and rush told you that this word is retired. Let's see what replaces it. There is one spelling for starting work on a process: process dot spawn. To dispatch a worker, write process dot spawn with run, a colon, and the worker's call. That one verb covers it. Process dot run was a second spelling for the same act, and the rule is one spelling per thing. Now, how does the result come back? As a letter. When the worker finishes, it sends a message to you, with an answers field holding the id of the letter you sent. And if you only want to read the result and wait for it, process dot await is still alive, and does exactly that. So the old pattern, run and get the result, becomes spawn the worker, then either read the answer letter or await it. Rush says retired by name instead of not found. And if you wrote your own function with this name, yours wins. Replace run with spawn, and your worker starts.
Text from the Rush engine's error catalogue.