OP:
Are you still getting nowhere?
May I offer another suggestion?
Try downloading the Low Sierra (10.12) installer using this link:
Download macOS Sierra
(I believe this is a "direct link" and will immediately download a .dmg file to your Mac)
Open the disk image and you will see a "pkg" file.
Double click that and see if the installer opens.
Are you still getting nowhere?
May I offer another suggestion?
Try downloading the Low Sierra (10.12) installer using this link:
Download macOS Sierra
(I believe this is a "direct link" and will immediately download a .dmg file to your Mac)
Open the disk image and you will see a "pkg" file.
Double click that and see if the installer opens.