How to use barman to backup PostgreSQL database correctly?
When pg_receivexlog was renamed, barman didn't add sprinkle conditional error messages all over its code.  It is looking for pg_receivewal, but the text of the error message still says "pg_receivexlog" as an umbrella term to cover whichever applies.

The solution is to add the directory holding pg_receivewal to your $PATH.  It should already be there, but apparently it isn't.  How did you install both PostgreSQL and barman?  Which OS user are you running this command as?