I erased the disk and on start up there is a folder with a question mark flashing.

If a flashing question mark appears when you start your Mac - Apple Support

On startup...

Command (⌘)-R Install the latest macOS that was installed on your Mac.

Option-⌘-R Upgrade to the latest macOS compatible with your Mac.

Shift-Option-⌘-R Install the macOS that came with your Mac, or the closest version still available.