Hi, I'm David an independent advisor and user just like you. I'll do my best to help you with this.
You've probably converted your drive to UEFI boot format but UEFI mode is not enabled in your BIOS (or even worse not support by your BIOS). You need to try and enter the BIOS settings page... hopefully before the text you've mentioned above there is another page and it might show instructions for how to enter the BIOS. Often you would press DEL, or F12, or F2 or similar - see whether you can see any help on the screen.
If you can get to the BIOS look at the boot options and check whether you can enable UEFI boot mode.
Regards,
David