convert ssd hard GPT to MBR?

Hi Ali gh,

If you're referring to an external drive (not the internal drive inside your Mac), you can use MBR as the drive scheme using these steps:

WARNING: These steps erase all data from the external disk. Back up any data on the disk to a safe location off the disk first.

1. Open Disk Utility, located in Applications -> Utilities.
2. Click View -> Show All Devices.
3. Select the top level of the external drive and click Erase.
4. You should now be prompted to select some options. You can configure the name and format as you wish. Set the Scheme option to MBR.
5. When ready, click Erase to reformat the drive.

If you're attempting to use MBR on your internal drive, this is not recommended. macOS runs best when the internal drive uses GPT. Additionally, using FileVault (as far as I know) requires the drive to use GPT.

https://support.apple.com/boot-camp

Caution - doing so will wipe the Internal Drive of everything including the Recovery Partition. This is a One Way venture and no reasonable way back to using macOS.