unable to erase disk on my disk utility

You cannot erase the drive you are booted into.

You need to boot into recovery - follow these directions

https://support.apple.com/en-us/HT201065

cindylk2 Said:

"unable to erase disk on my disk utility: I can't erase my disk in disk utility - I am prepping to send my MacBook Pro for trade in...it is grayed out when I click on mackintosh HD"

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Mount the Drive:

Your drive may no longer be being seen, for what ever reason. So, boot into Recovery Mode (Hold Down: [Command + R] upon boot). Then, using Disk Utility, see if you need to click the Mount button for it. Then, try rebooting once more. If of no success, then click the Unmount button, and then the Mount button. Try rebooting once more.