recovering abandoned laptop, bitlocker issue

Normally you should be able to remove Bitlocker encrypted partitions through this menu, but perhaps it's being a bit stubborn. Let's try this instead.

Instead of clicking the install Windows options in the Windows installer menu, click repair your computer instead. From here click Advanced options and get to a Command. Hopefully it will not prompt you for any passwords or keys as part of this process. it usually doesn't when you're doing so from USB media, I never know. Once it opens, type diskpart and press enter.

Once your the computer launches diskpart in the command line window you'll get the diskpart> prompt.

Enter list disk The command will output a list of the current disks connected to the machine, including your USB drive. Your boot disk will probably be disk0, but whichever disk it is select that disk by typing select disk N where N is the number of your internal disk in the output.

I might try typing clean here, and see if that alone works, but it may not. If it doesn't you can select the individual partition and delete that by typing list partition then select partition N in the same way you did for the disk, where N is partition number you want to delete. Then type delete partition override. If you don't get any errors, type exit, restart the computer back into your install USB and see if that helped.