I installed Ubuntu-20.10 on my computer. So now there are two operating systems on my computer, Ubuntu and Windows 10. But I turned on the computer and found that the computer automatically entered the Windows operating system without entering the selection interface of operating system. And there is no Ubuntu startup item on the boot option in the BIOS.
My guess would be the installer didn't make a boot partition. Could be allot of reasons. Does your bios have UEFI ? some bios will launch a usb installer not from UEFI with can screw up the installer. other times it's just a wrong button pressed. usually people have the opposite problem. Make sure your bios is up to date and when installing again check the partition summer before you hit install to make sure its making a bootable partition or including it somewhere.