This article is about imports and exports in newly created database. There is the way to fix this error, but I choose the easiest solution as I could do with database anything I'd like. Article from the link you could use when you can't drop database, but you need to do import.

The problem is in source and target database charset. Check it with sqlplus on both databases with command. There are several command for charset checking, but this one clear and effective.

SELECT value$ FROM sys.props$ WHERE name = 'NLS_CHARACTERSET' ;  

It will differ and you've found what problem is. So you should recreate database using this article and make source and target charset equals.