Select your hard drive from the left column in Disk Utility.

It is most likely the uninitialized drive.

If you have multiple hard drives, make sure you select the correct one as the next step will wipe the selected drive clean.

Click the "Erase" button at the top of the window.

Choose a name for your drive (you can rename it later). Leave the scheme as GUID Partition Map.

If you are installing an SSD, choose the APFS format. If you are installing a mechanical drive or if your device doesn't officially support the newer OS, leave the format as Mac OS Extended (journaled)

Click "Erase." When the process completes, click "Done."

Exit the Disk Utility by clicking on the red X button on the top left corner of the window.