Oracle create database link and error ORA-12514

Creating a database link in Oracle is pretty straightforward and both the official documentation and Toad's Knowledge Xpert well detail the steps.

What might happen afterwards however, is that you successfully test your DB link with a client like Toad or SQLPlus, but get the ORA-12514 "TNS: listener does not currently know of service" error when using it from other clients.
This lengthy post goes well deep into how to troubleshoot errors like it, but the most basic thing you should always check first is "Are my TNSNAMES.ORA files correctly configured on both the local and remote DB?"

You may find that the reason why the client you tested the DB link with worked but the link does not work from another source instead, is that your client uses its own copy of the TNSNAMES file while the one read by your DB server is not correctly updated.