When I create a package in package manager, I build and install it, after which I download the package, but that gets downloaded a folder instead of a zipped file. Next if I try to upload the same page again after zipping it, it shows me error: as given in the image.
OOTB Package manager provides only .zip if your Mac/OS decides to unzip it, it's a matter of you to config OS in a way that package won't be unzipped or zip the folder back correctly, without loosing essential info for package manager system.
Infact, the problem is really peculiar. At times when I download a package, I get zipped file, which I can upload. But, at other times when I download, I get unzipped file, which on compressing doesn't gets uploaded!
When I get the downloaded folder I created a zip using the jcr_root and meta_inf folders, only that zip get uploaded successfully. But if I try to zip the folder itself, which contains the jcr_root and meta_inf, that doesn't get uploaded.