I would like to download an installable copy of High Sierra
I am unable to download it from the APP Store. I get an error message
iMac 27″, macOS 10.13
Question marked as
What is the exact error you are receiving?
What is the exact version of your Mac and current version of macOS being used? You can get this information by clicking the Apple menu & selecting "About This Mac". FYI, it is usually best to include this information whenever you are asking for assistance online.
How much Free storage space do you have on your boot drive? Ignore the very misleading "Available" storage value shown everywhere within macOS. The Free space value is only shown in Disk Utility & the System Profiler (Disk Utility is easier to access that information). You always need to keep at least 20GB+ (or better yet 20%+) of Free storage space at all times just for booting macOS (that space can disappear quickly even with simple tasks). To download the High Sierra installer requires about an additional 8GB of storage.....so an absolute minimum of 30GB of Free space is required.
If your Mac you are using to attempt downloading High Sierra is compatible with High Sierra, then try booting into Safe Mode to see if that makes a difference.
If you already have a High Sierra installer in the Applications folder, then you may want to delete it & remove it from the Trash. Then try again to download the High Sierra installer.