When I type a passphrase with only letters and spaces, I get

  A passphrase should contain at least 1 digit or special character.

probably because the default --min-passphrase-nonalpha value is 1.
On a long passphrase, this doesn't add any security (in particular,
adding one random letter provides more possibilities than a random
digits) and this is against NIST rules

https://pages.nist.gov/800-63-3/sp800-63b.html#memsecretver

"Verifiers SHOULD NOT impose other composition rules (e.g., requiring
mixtures of different character types or prohibiting consecutively
repeated characters) for memorized secrets."