I used the utility to erase my disk and now i want to reinstall Mac OS. It says that the disk is not formatted. How do i format the disk

As you are posting in the High Sierra community I am assuming your mac is running macOS High Sierra or was.

It might have helped if you bothered to include information on what model and year your mac is,

what OS you were running and what OS you are hoping to reinstall on your mac.

Make sure your mac is connected to your router via cable rather than WiFi.

Restart your mac while pressing and holding down the Command and R keys.

This will restart your mac to the Recovery HD.

You should see a Utilities panel.

Select Disk Utility press Continue.

Click on view in the menubar and select Show All Devices.

click to highlight the Disk not any of the indented Volumes.

The Disk is possibly called Apple SSD…

Click Erase.

Give the Disk a name,

Format: Mac OS Extended (Journaled)

Scheme: GUID Partition Map

Click Erase.

When Done quit Disk Utility

Click on Reinstall OS press Continue.

The mac should connect with Apples Servers and download the OS your mac came preinstalled with.