If you’re trying to register a free domain on freenom.com (you obviously are) and get the message:
Attention! Some of your domains could not be registered because of a technical error.
Don’t fret! Here’s a quick solution:
Change the network you’re using to access the internet.
In my case, I was using a mobile hotspot (from internet service provider 1), I switched to another mobile hotspot (from internet service provider 2) and voila, success!