Trouble after erasing disk

As I say, it likely wasn't formatted correctly.

For mid-2011 or later Macs, restart holding the command-option-shift-R key sequence. You should get the option to reformat the drive with disk utility. Once the formatted proper, quit disk utility and run the installer. The Mac App Store should get you to 10.12, and then you can upgrade to 10.13 if it is compatible.

Fo March 15, 2010 to pre-mid-2011 computers, find the original installer disc. AppleCare may have it. Boot the disk with the Option key, and then open from the menu Disk Utility to reformat the drive. Run the installer, and run the combo update for 10.6.8. Then run the 10.11 installer from the Mac App Store, then you can try to get to either 10.12 or 10.13 if the Mac is comaptible.

For pre-March-2010 computers, get the 10.6.3 retail installer disc that looks like:

Directions are otherwise similar to the March 2010 to mid-2011 computers.

Use http://checkcoverage.apple.com/ to find out the age of the computer, after finding the serious number on its bottom or rear. Use a magnifying glass if you can't read its fine print. A cell phone zoom photo lens can also help.

https://discussions.apple.com/docs/DOC-3003

Formatting with Disk Utility during the erase affects the ability of certain systems to boot. Don't use case sensitive systems, or Windows file systems if you want to boot Mac OS X.