Since Oracle database 19c, we have a new UI of Enterprise Manager Express (EM Express) which is based on Java JET technology.
As you can see, some functions we used in EM Express 12c are not available anymore in the new UI. On the positive side, it responds very fast, very express. But maybe it's too simplified to be useful for DBA. Luckily, we can revert the UI back to flash-based.
SQL> @?/rdbms/admin/execemx emx
no rows selected
old 1: select nvl( '&1','omx') p1 from dual
new 1: select nvl( 'emx','omx') p1 from dual
PL/SQL procedure successfully completed.
If you don't know what @ or ? means, you can check the post: What Symbols are Used in Oracle Database.
After that, please re-logon to make it work.
To go back to Java JET based UI, you can do this:
SQL> @?/rdbms/admin/execemx omx
For more information about EM Express 19c, you can refer to: Introduction to Oracle Enterprise Manager Database Express 19c.