Incompatible Disk Error after Mojave install

Did you downgrade from macOS 10.15+ by any chance? I believe this may be an error where Mojave sees the old "Update" volume utilized by macOS 10.15+ and indicates you did not completely erase the whole physical drive (Intel Macs only) when reinstalling macOS. It may also mean you have other APFS volumes consuming storage space as well. If you downgraded macOS, then use the following command in the Terminal app so we can see the current drive layout.

diskutil  list  internal

Thanks for the follow up. I believe my guess was most likely correct then. You may still want to look into the current drive layout though since you may have an APFS volume which is wasting some storage space. Many times when people reinstall macOS, they end up with an extra volume from the old system. If it is the "Data" volume, then a lot of storage space may be taken up by it.

If it is a 128GB SSD, then usually you only get about 100GB to 120GB of space from it for macOS once it is formatted.

Also, the "Available" space designation within macOS is very deceptive since it does not mean that storage is immediately useable. The only way on macOS to find out the actual amount of storage space immediately useable for your files is by using Disk Utility & selecting the "Data" volume on the left pane while looking at the "Free" space value on the right side just beneath the bar. The "Free" space value is storage you can use immediately. The "Available" designation contains both the "Free" space plus "Purgeable" space which is storage macOS will release at some unknown time in the future..

"diskutil" is a command line utility which provides for some of the functionality of the Disk Utility GUI app. "diskutil" is mainly used to create & resize partitions, mount/unmount volumes, and to "format" volumes. The Disk Utility app is very clumsy and will hide a lot of stuff from the user, the command line is able to reveal a lot more which is important when dealing with install issues & space issues.

FYI, on *nix systems (macOS, Linux, BSD) with the command line, you can gather more information about most commands by reading their manual or "man" page which includes the proper use & format of the command & its available options. For example, to get information on "diskutil" you would use:

man  diskutil

Or if you want information on the "man" command:

man  man