Have you been able to chroot, either with the relevant button in the Garuda welcome or with this tutorial?
If so, when inside chroot, check that /boot/efi directory exists. If not, create it. Then /dev/sda1 is ok to be mounted there.
Wait, the method above is valid if you still have a partition where you tried installing but the process failed at the end. If you deleted your btrfs partition, that won’t apply.