Truncating a table in a stored procedure
All DDL statements in Oracle PL/SQL should use Execute Immediate before the statement. Hence you should use:
execute immediate 'truncate table schema.tablename';
As well as execute immediate you can also use
DBMS_UTILITY.EXEC_DDL_STATEMENT('TRUNCATE TABLE tablename;');
The statement fails because the stored proc is executing DDL and some instances of DDL could invalidate the stored proc. By using the execute immediate or exec_ddl approaches the DDL is implemented through unparsed code.
When doing this you neeed to look out for the fact that DDL issues an implicit commit both before and after execution.