Here's why you can't use GTS on Diamond and Pearl anymore:
* NWFC is discontinued: The service that allowed online play on those games no longer exists.
* No alternative: There isn't a replacement service or emulator that can replicate the NWFC functionality.
What can you do?
* Local trading: You can still trade Pokemon with others locally using the DS's wireless connection.
* Emulators (with caveats): Some emulators allow for online play, but it's not officially supported by Nintendo and may be considered piracy in some regions.
* Fan-made communities: Some online communities offer unofficial trading systems, but these may have security risks.
Remember: Always be careful when connecting to unofficial services and ensure you understand the potential risks before using them.