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got kubuntu on imac installed trying to change os to os x dont have other mac devices for bootable drive and tryed internet recovery but said apple.com/support -21 06F
Sadly you do need to create a bootable OS installer USB thumb drive. From a friends Mac system follow this guide How to create a bootable macOS Sierra installer drive Apples certificate has expired in the posted OS’s so you’ll need to go here to get the one that has the updated certificate How to upgrade to macOS Sierra Jump down to Step 4 to get get to the link. Here’s a bit more on the subject If you’ve got an old macOS install image, it will probably stop working today I strongly recommend sticking with Sierra as High Sierra has issues with APFS
