There isn't a ready to run image available. You can make one with Ayufan's build scripts or you can grab the RockPro64 minimal image, loopback mount it, mount the filesystem, chroot into it (with a network connection available) and "apt install linux-pinebookpro-0.9" -- that'll get you a bootable minimal command-line system image that you can flash to an SD card after you unmount the filesystem and loopback. You can burn the base image to an SD card and chroot into that too instead of using loopback. You'll probably also want to patch a U-Boot into the image that supports sd booting if you're going to flash it to the eMMC, otherwise you won't be able to boot directly to an sd card after you write the image to your eMMC.
I've been meaning to write this up as a how-to and provide some scripts this week but between the holiday and a nasty cold I haven't had the brainpower to write up a guide. Anyway, the above will get you an arm64 userland 18.04 system, you'll probably want to install any missing tools packages (wireless tools, etc) that the RockPro64 doesn't need by default and test it from an SD card to verify that it's got everything you need before you write it to the eMMC.
