How to override ‘o install Windows 11?

Hi, I’m Rodrigo and I will help you.
VirtualBox is known for some incompatibilities on the Windows 11 upgrade, that’s why it shows this message.
Since you said you can’t find VirtualBox to uninstall it, there are other virtualization applications that can trigger this same message.
If you don’t know or don’t use any, another option is to disable the Virtualization on Windows and/or your BIOS.

To do it on Windows, go to Settings > System > Optional Features > More Windows features (in the right side) > Make sure the “Hyper-V” and “Windows Hypervisor Platform” options are unchecked.