@Ffiona5
If this is a fairly new AIO, within a couple of years old, it came with Windows 10 preloaded in UEFI mode with Secure Boot enabled.
This message generally means someone has messed with the BIOS settings and is NOW, trying to reboot with Secure Boot disabled but still in UEFI mode -- and from my experiences, that will not work.
If you have not messed with these settings, since we here have no access to your PC, you will need to take it to a local PC repair facility to have them check it out and restore the proper settings for you.
I am a volunteer and I do not work for, nor represent, HP