You should not disable Secure boot, nor enable legacy support for Ubuntu to work.
Go into BIOS and reset to default, I believe F10 does that. After powering on tap F9 to select boot device. If this doesn't work, then remake bootable ISO using UNetbootin.
Making Bootable USB using UNetbootin
If using Windows, run the file, select an ISO file or a distribution to download, select a target drive (USB Drive or Hard Disk), then reboot once done. If your USB drive doesn't show up, reformat it as FAT32.
There has been some confusion between Distribution and Diskimage and I am going to clear the uncertainties before hand.
There is a Radio button with the text Distribution next to it. If you don't have a Diskimage then you would select this option so the image would be downloaded.
On the other hand, there is another Radio button at the bottom with the text Diskimage next to it. You would select this option if you have an image ISO file and you don't need to download an ISO.