There are a number of reason your Raspberry Pi may fail to boot from the PiNet server.
Note the video above includes hard written transcriptions which may help with some commands.
If you get a blank screen or a rainbow screen, this is most likely to be 1 of 2 possible issues
If the SD card does not have a FAT partition as the 1st partition with all the generated boot files, the Raspberry Pi will fail to load. The fix for this is reformat the entire card. See the SD card guide for how to do this.
If the SD card is formatted correctly, but the boot files are corrupt or missing, it will also fail to boot. Check the config.txt file and check the files defined after kernel and initramfs exist on the SD card root. If in any doubt, copy the entire folder over from the server PiBoot folder again.
This usually means it can’t correctly connect to the PiNet server. This can be identified by if the console shows Giving up errors or if it drops out into an (initramfs) prompt.
As the server IP address is saved in the cmdline.txt file on the SD card, make sure it is correct. You can view the server IP address by opening PiNet. It is displayed in the top right. Check that this is the same as on the SD card in the cmdline.txt file. If not, change it to the correct address.
If you are dropped out into an initramfs shell, trying entering ping followed by the server IP address from above. You should get something like 64 bytes from... which means it is receveing a reply. If you get Request timeout for icmp_seq... then there isn’t a direct link between the Raspberry Pi and the server. Check your cables. Is there perhaps some sort of firewall in the middle?
On the server, open a terminal and enter ls /opt/ltsp/armhf.
If you see a folder titled debootstrap then this means the PiNet software failed to correctly build Raspbian. This usually happens because the internet drops out half way through or your network is blocking access to part of the Raspbian repositories.
To fix this issue, it is highly recommended you start from the start again, reinstalling Ubuntu then PiNet to make sure everything is cleared out.
If it continues to fail, try installing PiNet using a different internet connection, perhaps try taking the server home?
If these suggestions don’t help, you may want to contact support.