On dual-boot macOS/Ubuntu Mojave install failed with "This disk cannot be used to startup your computer."
I deleted the Ubuntu partition and then used macOS utilities, Disk Utility and diskutil to recover that space. Then the upgrade to Mojave proceeded normally. At that point I used these utilities to resize the Macintosh HD partition to half the disk and created a following spacer partition of 128 MiB (see rEFInd forum). I then reinstalled rEFInd from macOS and Ubuntu in the remaining free space. The result is a working Mojave/Ubuntu dual-boot system.