How to fix This PC's processor doesn't support a critical feature SSE4.2?

"This PC's processor doesn't support a critical feature SSE4.2"? Unfortunately, there's no way to fix a processor's lack of support for SSE4.2 or PopCnt.  These are fundamental hardware features.  Windows 11 requires them for proper operation.

Your options are limited, and boil down to:

Don't upgrade: The most straightforward solution is to not upgrade to Windows 11.  Stick with Windows 10.  This is often the best option if you value stability and compatibility above all else.

Consider a different operating system:  If you absolutely must use a newer operating system, consider Linux.  Many distributions are quite capable and may be a better fit for older hardware.

Use a Virtual Machine (VM): If you're just curious about Windows 11, you can install it in a virtual machine.  This way, you won't affect your existing Windows 10 installation.  However, performance in a VM might be significantly lower than on native hardware.

Why in-place upgrade attempts often fail:

In-place upgrades, especially when trying to move from one major OS version to another, are notoriously prone to issues like this.  The setup process isn't designed to fully account for all possible hardware configurations, and it can often get hung up on these fundamental hardware limitations.

In short: When you encounter the issue "This PC's processor doesn't support a critical feature SSE4.2" Your processor is simply not capable of running Windows 11.  Don't waste time trying to force it.