Please provide the following info which will help pointing out the issue:
- Detail Oracle APEX Version (if you are configuring ORDS for APEX)
- Detail Oracle Database Version
- Detail ORDS Version
- Detail Supported Java EE Application Server Version (if using one)
- Detail JDK Version
NOTE : Here "Detail" means the major+minor version. For e.g. Application Express 4.2.4.00.08
For debugging this issue you are facing on ORDS:
- Locate the ORDS configuration folder.
- Enable the Debug Tracing in defaults.xml (If entry is not there add the entry)
- Enable the Detail Request Error messages in defaults.xml (If entry is not there add the entry)
- Start the ORDS in standalone mode / Start(re-start) the Java EE Application Server.
- Navigate to the APEX URL
- Copy/paste the debug log into the forum reply.(Usually the error is at the end of the log, paste/attach the whole log from where the first exception occurs)
Also have you configured the "PL/SQL Gateway" while configuring ORDS (viz. APEX_PUBLIC_USER configuration) or have you skipped it?
In addition to the things mentioned above check whether the REST users viz. APEX_LISTENER and APEX_REST_PUBLIC_USER have CREATE SESSION privilege.
Refer following similar thread : Re: ORACLE REST DATA SERVICES 404 NOT FOUND