The current message text is: "You cannot block anonymous users only when blocking a user."
This does not parse properly, or at best is worded confusingly. The existing translations of this message generally do not make any sense.
Documentation says: "Used as error message on Special:GlobalBlock if the blocker tries to globally block an account when they have specified anon-only."
So I suggest rewording this message to: "It is not possible to restrict blocking to anonymous users when blocking a user."