dual boot – Try to completely reinstall the new version of Ubuntu

I am a little lost.

So, I have a dual boot Win10 / Ubuntu 14 (I think), but it's getting old. The Ubtunu score is basically made for, I had to give Frankenstein more space for a computer project and he now thinks that it is encrypted. In addition, I have a connection loop problem and I have not been able to solve it.

I am now on vacation, so I have time to do a relocation, and I would like to erase everything and start over. I have a USB key with Ubuntu 18, but I have some problems: I tried to boot from the GRUB command line (as a result of this great tutorial that worked once ) but I am stuck at the point search --fs-uuid --set = root UUID step. I get "failure to read sector 0x0 on partitionX" and "none of these devices: UUID" (I'm sure I've got the right UUID since ls (partition) give it to me and tell me that it is Ubuntu 18.

I have therefore tried to work around it and use the BIOS to try to boot on the USB, but I have an Acer and I do not know how it happened, but he now has a password that I do not know (I've been looking for this and I've seen no solution that does not require me to dismantle this damn thing).

Do you have an idea of ​​what to try next?