PL/SQL Developer: Can't Connect to Database Error

Right after you install PL/SQL Developer, you realize that you cannot connect to your databases. You get the following message:

ORA-12154: TNS:could not resolve the connect identifier specified

For a moment, you may think, it is something to do with TNSNAMES.ora file or Pinging issue. It is, however, purely a PL/SQL Developer issue. When I downloaded SQL Developer from Oracle and installed it, I could connect to all my databases fine.

The solution is actually pretty simple. There are probably multiple ORACLE_HOME defined (such as for reports and forms, workflow, discoverer, etc.) in your machine, and PL/SQL Developer is probably not finding the correct one to choose from. All you have to do is to choose the correct version for your PL/SQL Developer. To do so, follow the below instruction.

1. Make sure you are connected to Network/VPN.
2. Open PL/SQL Developer.
3. Click on Tools -> Preferences. Select the very first option: Connection.
4. From Oracle Home drop down list, select the correct version. For example, OH159997875 works for my machine.
5. Restart PL/SQL Developer.
6. Cross your fingers and retry to connect to your databases.
7. If it works, say "Buiya". If it doesn't, scream "F***", and Google search.