"/achievement give" isn't working.

Usually, "/achievement give" is supposed to let you get an achievement without actually doing it.

But when I typed: "/achievement give onARail [My username]" it said:
Unknown achievement or statistic 'onARail'

Steps to Reproduce:
1. Type: "/achievement give [Achievement name] [Player name]"
2. Then, chat says: Unkown achievement or statistic 'onARail'