SQL> create table test1(c1 nvarchar2(2000));

Table created.

SQL> declare
  2  str1 nvarchar2(2000);
  3  begin
  4  str1 :='insert into test1 values(''testing purpose'')';
  5  execute immediate str1;
  6  end;
  7  /
execute immediate str1;
                  *
ERROR at line 5:
ORA-06550: line 5, column 19:
PLS-00382: expression is of wrong type
ORA-06550: line 5, column 1:
PL/SQL: Statement ignored