How to dual boot? bitlocker encrypted.

As you can see in the screenshot, Windows 11 is already installed on the C: drive.
I want to install another OS. Where can I install it without having to take BitLocker into account?

If you have another drive or partition such as the D in the screenshot, you should be able to install there. Only the partition the Windows partition is encrypted. An alternative would be to run your second OS as a VM. Less hassle and less opportunity to break things.