error code during diagnosis

If that is the complete error message, then @Flannelist is correct that it is a network issue. Try connecting the laptop directly to your router for a faster more reliable connection that bypasses any possible WiFi issues which are hard to troubleshoot. You can also try a PRAM Reset as well since sometimes a Mac will retain an incorrect link in NVRAM after a failed network boot.

If you still have this error, then try power cycling your router making sure to wait five minutes for the router to finish booting.

If the error includes something like "unable to find EFI/Support.efi", then this is a different error caused by Apple when macOS updated the system firmware and which Apple decided to never fix since the firmware can no longer locate the boot file on the Apple server.

Hi UPO-DK,

Thank you for using the Apple Support Communities. If you get an error code when you run Apple Diagnostics you would want to follow the troubleshooting steps in the article below to try and isolate your issue. 

About macOS Recovery on Intel-based Mac computers

We hope this helps!

Take care.

What issue are you trying to run diagnostics for? If you're getting this error, it's trying to connect to internet based diagnostics as the local diagnostics information can't be reached (or doesn't exist). And it can't load this. Many times, it gives errors because it's timing out, the network isn't fast enough to load the image. I would reset NVRAM/PRAM https://support.apple.com/en-us/HT204063. And try again, use a wired network if you are able, rather than wifi.