Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
Rush just told you it does not know a filesystem you named. Here is why. On a filesystem holder, the word which tells rush what kind of disk your data rests on. Rush does not take that word on faith. It checks it later against what the kernel reports, and to do that it must recognize the word first. The words it knows are leafs, ext4, btrfs, xfs, tmpfs, ntfs, and apfs. Anything else, say a typo like ext five, or a name from some other system, names nothing rush can verify. So it refuses, naming your holder, the word you wrote, and the full list of words that would have worked. Picture writing which colon ext3 because that is what your old server used. The message will list the legal words, and you can pick the closest one, or leave out which altogether and let leafs be the default. This protects you from a quiet mismatch, where you think the data is on one kind of disk and it is on another. Read the second slot, choose a word from the list, and rush will continue.
Text from the Rush engine's error catalogue.