You don't show really your actual code/files, so hard to tell you what's wrong, but it's likely you fucked up something.
To get this error, it means either the language file is not parsed, or key has not been found.
Since file exists and key exists (at least what we see from first post), it sounds like you don't test properly or did something like renaming data folder or similar.
Anyway, you can have more informations if you set amx_mldebug to 1, then restart ; it will throw messages in amxx error logs if something is wrong.