title: how to resolve SQL Error: ORA-14411
body: <p>I am trying to alter a table by adding a column, but it's giving the following error:</p>
  
  <pre><code>ALTER TABLE TUSER
    ADD CREATED_BY VARCHAR2(250)
  
  SQL Error: ORA-14411: The DDL cannot be run concurrently with other DDLs
  </code></pre>
  
  <p>How to unlock the resource which is causing this error?</p>