But, your picture showed a TM disk, judging by the icon, so that's not just another disk, it's one used for a Time Machine backup.
Are you sure you are selecting your destination disk (the one that you want to have as a boot drive, with your newly installed system)
A tip to help you make sure that the drive you want to erase is the correct disk: Change the name of the correct disk before you try your next erase. I usually use a ridiculous name, like "malfactorActor", or something you can choose on your own as a really stupid name for a storage or boot drive.
You're erasing it anyway, so the name would immediately change to something else during the erase ("Untitled", if you don't enter a new name yourself).
And, the correct drive would now be erased