Please, review other threads in this forum and see what questions people are usually asked when they pose questions with few details. The database character set would be the most useful information that you could provide.
The definition of the function utl_i18n.get_default_charset() says:
This function returns the default Oracle character set name or the default e-mail safe character set name from an Oracle language name.
This is not the database character set name. The database character set name is returned by SELECT value FROM nls_database_parameters WHERE parameter='NLS_CHARACTERSET';
Please, check chapter 2 of the Oracle Database Globalization Support Guide.