Help with OS reinstall on Thinkpad Carbon X1

Hi, my father in law got a Thinkpad Carbon X1 a few weeks ago. Apparently he went though initial setup but now does not remember the pin to login.

I have tried to hold SHIFT and rebooting, which gets you into a special recovery node. Reinstalling Win from here wont even start.

I have downloaded Win 11 install media from MS on a USB and booted to that but I cannot install on the big partition, as apparently BitLocker is preventing it.

Does anyone know the best way to get a fresh install of Windows on this machine.