'grub-efi-amd64-signed failed installation /target/' even though I have rEFInd (Ubuntu 18.04 dual booting on macOS High Sierra)

I have a macbook pro (2017) with macOS High Sierra and am trying to dual-boot Ubuntu 18.04. I installed rEFInd and was able to run Ubuntu from a live USB. However, when I went to actually install Ubuntu permanently to a partition created for it, I went through several steps of installation only to run into 'grub-efi-amd64-signed failed installation /target/'. The installation then stopped.

A lot of online posts suggest that I download rEFInd, but I already have it - I believe that's how I choose which OS to boot even when using the live USB. I haven't been able to find help on what to do given that I have rEFInd, and I think that Ubuntu should be installing smoothly. Any help would be appreciated.