You may run into this error when attempting to partition your drive for a new Ubuntu setup, reinstall, or dual-boot. After attempting to commit your partition edits, a red screen appears with the following error:
“No root file system is defined”
This means you didn’t properly configure a partition as your root file system. Go back to the partition manager and make sure at least one partition is bootable, is in use as a file system, and has the file system usage set to “/”, or root. This makes the partition the default partition for your Ubuntu install’s binaries, data, etc.
Once you set a root file system partition, the installer will allow you to continue.