Can't delete the main Mac hard drive I want to downgrade from macOS Sierra to OS X Mavericks. I have backed up my Mac using Time Machine and I made a USB with the OS X Mavericks Installer.

The thing is that when I reboot my Mac with Alt/Option, enter the USB Installer and open Disk Utility I can't delete the main drive since there isn't just the button to erase it, but the volume ("Macintosh HD") has this button. How can I solve this problem?
If your volume is 'formatted' with coreStorage as a logical partition, reverting might be enough to resurrect the "Erase" tab.
In Terminal run the following command:

diskutil cs list


Note the disk identifier for the logical volume (e.g. on a Mac with only one drive and nothing else attached, it would be something like disk1)

Now try and revert from coreStorage (replacing disk1 with your own found disk identifier):

diskutil cs revert disk1


(you can run this either booted from your usual volume, or the installer disk)

If successful it will be mentioned in Terminal and the "Erase" button should have returned in Disk Utility, when you're booted from your installer drive.
(With more drives connected, make sure the disk identifier is the right one for the logical volume you want to revert)

If you can't successfully revert, make sure File Vault and encryption have been disabled, then try by destroying the whole coreStorage group with this command:

diskutil cs delete lvgUUID


replace lvgUUID with the UUID you found earlier in the diskutil cs list command under "Logical Volume Group".