Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
You named a key with digits only, like one or two, and the engine said a key needs at least one letter. Here is the reason. In rush, when you write m dot one, that always means position one. So if a key were named one, you could put the value in, but you could never read it back by name, because the name would be understood as a position. The key would be unreachable. To avoid that trap, a key must be text with at least one letter. So item dash one is a fine key. It has letters, and it reads clearly. A bare one is not. This refusal used to say that the parser did not understand a construct, which blamed the wrong party. The parser was fine. The law about keys was the one speaking. Now the message shows the key as you wrote it, and suggests the fix. Add a letter or a word: item one, first, or whatever describes the value. A good key says what its value is, and now it also can be found again.
Text from the Rush engine's error catalogue.