l can't enter BIOS because Ubuntu loads too fast
Solution 1:
The warning message you got is not important. You can ignore that one.
Your problem will not be solved, when grub takes longer to choose. You first start with your BIOS layer, then you will get to grub and grub loads the Linux-kernel and hence Ubuntu. When you have reached grub, there is no way back into the BIOS Setup.
But in GNU/Linux you can reboot directly into the BIOS Setup, by the following command:
systemctl reboot --firmware-setup
I found the command on Super User.