If Windows was installed to run in UEFI, the Linux OS must also be installed using the UEFI mode. This means that most likely you need to disable in the BIOS the legacy mode and allow secure boot.
Is the "Install Ubuntu alongside Windows 10" option missing?
Please note that in the following circumstances the Ubuntu 20.04 installation option "Install Ubuntu alongside Windows 10" may be missing, if your Windows 10 installation: is not correctly shutdown or is hibernating, has corrupted partition which needs repair, partition has not enough free disk space left for resizing, uses Dynamic Disk or the file system contains uncontrollable file fragmentation.
Another case is if you try to install Linux using legacy boot for your installation media; the installation would be possible but the dual boot won't work.
References: LinuxConfig.org
