How can I erase Macintosh HD as it is greyed out as is the erase button on the disk utility, 2014 MacBook Pro

You can not erase the drive you are booted into. Follow these instructions: What to do before you sell, give away, or trade in your Mac - Apple Support

I have this Problem, and I understand that you can't erase a drive you a booted into, but what do you do when you use Disk Utility to erase your Macintosh HD, but the Macintosh HD-Data still remains and holding up 322GB of space which I am trying to free up.

boboyee2001 wrote:

I have this Problem, and I understand that you can't erase a drive you a booted into, but what do you do when you use Disk Utility to erase your Macintosh HD, but the Macintosh HD-Data still remains and holding up 322GB of space which I am trying to free up.

If this is an Intel Mac, then click "View" and select "Show All Devices" so that the physical drive appears on the left pane of Disk Utility. Select the physical drive which will be identified by the make & mode such as "Apple SSD....". Erase the whole physical drive as GUID partition and APFS (top option). This will destroy all partitions, Containers, and volumes on the drive. There is a chance you may need to delete the Data volume and agree to deleting the Container before Disk Utility may allow you to erase the whole physical drive.

If you have an M1 Mac, then the procedure is different as you should not erase the whole physical drive since a couple volumes are necessary to exist at all times.