According to the
Apple Support page pictured below, your screen shot indicates the actual hard drive is not normally shown. I assume that one or more of the names shown as "Externe" include the partitions you want to erase.
Is the "Save QuickSilver O..." selected? I can't tell by the grey background, sometimes a screenshot converts the normal background color to grey. Is that, perhaps, the name of the drive that contains all (or some) of the partitions?
I'm not sure if that OS allows you to add a partition to any other partition. You may have to simply wipe/erase the entire drive. Have you tried that? Partitions are often more trouble than they are worth.
Are you opening Disk Utility from the apps list or from your Recovery drive?Never mind, Mojave didn't have a Recovery drive.Erase a volume using Disk UtilityHave you tried your own suggestion: Erase them one by one.