Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
Your process call was shaped correctly, but the operating system itself refused to start the program. Here is what happened. Rush asks the kernel to launch your program, looking it up on the path, with nothing expanded. Its output goes into an anonymous file in memory. Most of the time, that works. This time the kernel said no. The reasons are usually plain: the program does not exist, or its name is misspelled, or there is not enough memory, or no file handle was available. Rush could have returned a gtrID anyway, but that identifier would belong to a process that never ran, and you would wait on a ghost. Instead it refuses, and it shows you the program in the first slot and the kernel's own reason in the second. So imagine you write python three but the machine only has python. The kernel answers: no such file. The fix follows the reason. Correct the program name, install what is missing, or free some resources. Then spawn again, and the process will have a real life.
Text from the Rush engine's error catalogue.