One of the more common Marvel Rivals errors is the “DX12 not supported” message, which halts players from enjoying the game to their fullest. The game mandates the use of DirectX12, so there is no workaround that involves switching to DirectX 11.
However, many modern systems that are capable of DX12 still seem to show this particular error message. Developers Netease Games have thankfully provided an official response (and fix), which you can find detailed below.
How to fix the Marvel Rivals “DX12 not supported” error

As explained in the official FAQ page of Marvel Rivals, players will need to update their Windows 10 installation to v1909 or later in order to be able to play the game. Windows 11 users should be mostly unaffected by this issue though.
Additionally, make sure your GPU meets or exceeds the minimum PC system requirements, and has DirectX12 support baked in. The issue can be caused by using outdated GPU drivers as well, so make sure to do a clean install of these updated drivers.
Potential, alternative workarounds

Assuming the official fixes listed above do not work in any way, you can try the following workarounds:
Reinstall your graphics card drivers
A bad Windows update can mess up your GPU drivers. Perform a clean installation of your graphics card drivers using DDU (Display Driver Uninstaller) and restart your PC to potentially solve the issue.
Verify the integrity of game files
Broken game files may result in a host of errors. Try verifying the integrity of your Marvel Rivals game install (Steam Library > Marvel Rivals > Properties > Manage Files > Verify Integrity of Game Files), and download any missing files if prompted to do so.
Consider upgrading your GPU
An outdated GPU will not support Marvel Rivals. DirectX 12 support is necessary in order to be able to run the game, and GPUs stuck on DX11 or older cannot run it.
Consider purchasing a new GPU if you do not meet the minimum system requirements.
Check if your are using your dedicated GPU
For those on hybrid graphics (such as on a laptop), try switching to the dedicated GPU over the integrated one. This helps boost performance and get rid of any strange bugs that may randomly occur.
Keep in mind that these fixes mentioned above are not universal, and you may have varying degrees of success with them.
Ultimately, no solution is truly bulletproof, and it might be better to wait until the developers release an official fix.
If you still encounter the DX12 not supported error, consider raising a support ticket to the Marvel Rivals customer service team.