Hello,
creating more than one topic for same problem will usually not help you to solve it.
You probably mixing libraries together or something similar. I see in your tree this - HTTPclient->LPC1768…
The "core/net.h" probably come from here and probably it will be only for LPC targets and it will be unsusable for microbit.
Probably you need this one
Edit: HTTPClient - mbed HTTP client | Mbed
BR, Jan