disk utility Problem

Is it the hard drive your computer booted from or an external hard drive? If it is an external drive, you may not have permissions. Try again from an administrator account. If it is the internal drive, you will need to follow a few steps.

First, boot into Recovery Mode by holding Command + R while your computer is starting up.

Next, click Disk Utility, then click erase. After entering a name, you should be able to click the erase button. This should erase your drive.

As 2399 said, you would want to boot into Recovery Mode. Then, open Disk Utility. If your drive is formatted as APFS, you will want to change the View to Show All Devices. Select the drive - not any of the volumes within - and you should be able to erase the drive. Or better yet click Partition to reformat it anyway you want (APFS, HFS+).