Because you installed rEFInd under macOS before installing Ubuntu, the refind-install script would have detected no Linux partitions, and would therefore have not installed any EFI filesystem drivers. Without those, and without GRUB 2, rEFInd will be unable to detect the Ubuntu installation.
To resolve the problem, you should install the ext4_x64.efi driver file. You can do this as follows (from memory):
- Boot macOS.
- Open a Terminal window.
- Run the
mountesp script that comes with rEFInd. This will mount your EFI System Partition (ESP), probably at /Volumes/ESP. (The script should say where it's mounted.)
- Copy the
ext4_x64.efi file from the rEFInd directory (wherever you unzipped the .zip file) to /Volumes/ESP/EFI/refind/drivers_x64/ (or adjusted for wherever mountesp mounted the ESP).
At this point, you should be able to reboot and rEFInd should detect the Linux kernel(s) from your Ubuntu installation. If you installed in one big ext4fs partition, rEFInd should be able to boot Linux at this point. If you installed using a separate /boot partition (because you used LVM or for any other reason), then instead of highlighting the Ubuntu entry and pressing Enter, you'll need to hit Tab a couple of times and add a root={something} specification, where {something} is an identifier for your root filesystem.