Fedora installation x86_64 ISOs use GRUB as the bootloader on computers with UEFI firmware, and isolinux (variant of syslinux) on computeres with BIOS firmware. My suggestion is to confirm whether the computer really is UEFI or BIOS; in particular you need to discover if you have a UEFI installation of Windows 10 or if it’s a BIOS installation of Windows 10.
Make certain Fedora is installed using the same firmware mode as the Windows 10 installation. This is committed at Fedora installation time, just like the Windows installer. That is, if you boot the installer in BIOS mode, you get a BIOS installation. If you boot the installer in UEFI mode, you get a UEFI installation.
If you determine that you should do a BIOS mode installation, I suggest trusting that the error may be correct, and just recreate the USB install media, possibly using alternate USB media.