I’ve heard issues with newer macOS this can happen as the virgin drive which is setup with High Sierra or newer which sets up a very different drive structure that the older Sierra macOS. Basically, this is related to the change from HPS+ to APFS file system.
So… if you had upgraded the remanence of he old would still be present.
You’ll need to reformat the drive removing all partitions, install Sierra in one partition then install Ubuntu in a second partition. You should be able to jump between using the onboard Startup Manager using the boot-up smart key service pressing the ***Option*** key.
Then you should be able to upgrade to Monterey and then use the Ubuntu dual boot option.