You may want to give a look at this guide:
but as explicitly stated there, and in many posts in the forum (please always use the “search” button), dual booting is not supported, and only good-willing people might offer their help.
And, to be honest, this looks to me like a strange case to troubleshoot.
If you have an EFI boot system, you should already have a fat32 formatted partition mounted at /boot/efi, and should be able to see that from the live USB with gparted (as shown in the guide) or using the terminal (lsblk or fdisk).
That partition should be used in the installation process, “partitions” step, install “bootloader on” at the bottom.