You can diagnose run-time SQL errors by looking up the ORACLE message for a given number. For example, if you get SQL error code 942, you can look up ORA-00942 to see that the table does not exist (or the user does not have any privileges to see the table).
A standard ORACLE server nstallation includes the utility oerr. To look up SQL error -942, use the following command:
oerr ORA 942
The ORACLE guides may provide more information than the oerr utility.