Dual boot: can no longer update to Mojave: "Disk cannot be used to start up"

I dual boot with Ubuntu. I have had no problem updating my system over the last 4 year (3 system updates). I now need to update to Mojave and get the error message This disk cannot be used to start up your computer. when trying to update. Would someone have a hint as to how I could do this without wiping out my Linux partition (as outlined in this thread )?