Can Somebody Help Me with Can't Convert GPT System Drive Back to MBR?

My problem is that the GPT SSD drive I want to convert to MBR is the current system drive hosting Windows, which cannot be cleaned/formatted, etc. Is there a way to convert it from inside Windows? If not, how to do it from outside? I have no other HDD/SSD to start up Windows.

Although the GPT disk is more sophisticated than the MBR disk, MBR is still the most compatible with an old Windows OS. That's why people prefer to use MBR sometimes. According to your description, I noticed that you want to convert the system disk to MBR. You don't need to manually clean or format the system drive before the conversion process. Converting GPT back to MBR is not as challenging as you thought, and many ways can achieve this goal. One reliable way will be presented to help you convert the GPT system drive to MBR.

EaseUS Partition Master is the top software that can convert the disk from GPT to MBR without erasing your data or partitions. It enables users to convert disk between MBR and GPT on all Windows systems, even without Operating System.

The following tutorial will teach you how to use EaseUS Partition Master:

Attention: Back up your important data just in case because this conversion process can harm them if anything goes wrong.

In addition to converting partition styles, EaseUS Partition Master can also support resizing and shrinking the partitions, creating new partitions, and so on. 

Note: Don't forget to change the UEFI boot mode to the Legacy BIOS mode in the boot settings. If you installed Windows 11 on your computer, you also need to enable the secure boot. Follow the below link to learn how to set up the secure boot:

Further Reading: How to Enable and Disable Secure Boot