It's almost always best to boot all your OSes in the same mode (BIOS/CSM/legacy or EFI/UEFI). Most computers with Windows 8 and later pre-installed use EFI mode for this, but a small mom-and-pop computer store might still use BIOS mode. It sounds like this may be what you've got.
In this case, the question is how to get the Ubuntu installer to boot in BIOS mode rather than in EFI mode. Unfortunately, the answer varies from one system to another. Typically, you can access the computer's built-in boot manager by hitting Esc, Enter, or a function key as it starts up. With any luck, you'll be able to find this easily, and you'll see two options for your boot medium, one with the string "UEFI" and one without it. Select the option that lacks the "UEFI" string and the installer should boot in BIOS mode.
If you can find the boot manager but you don't see a non-UEFI boot option, or if that boot option gets you to the same complaint you're seeing, you can try a couple of things:
- Create the boot medium in some other way. Some tools omit one boot loader or the other, so a BIOS-mode boot might fail and possibly fall back to an EFI-mode boot. Some tools (notably Rufus) offer options about what boot loaders to include. If you see such options, be sure to pick the ones for the most traditional BIOS-mode features, including the BIOS-mode boot loader and MBR partition table.
- Use another USB flash drive. Some drives simply refuse to boot on some computers in BIOS mode. I don't know why this is, but I've seen it on some combinations. Switching from a USB flash drive to a DVD-R is also an option, if your computer has a DVD drive.
As to the lack of the "install alongside" option, this might be a consequence of booting in EFI mode -- this option is usually absent when booted in this way. The solution is to use the "Something Else" option. Note that if you boot in BIOS mode, you may see this option appear. OTOH, it could be you've got a damaged or flaky partition table, or one with leftover GPT data. Some such problems can be fixed with the
fixparts tool, which is part of the
gdisk package in Ubuntu. See this page of mine and the FixParts documentation for more on this subject. (Note that I'm the author of
Oh, one more thing: Be sure to disable the Windows "Fast Startup" feature, which turns shutdown operations into suspend-to-disk operations. This can cause disk corruption, and could be another reason you're not seeing the "install alongside" option.