If you cannot get the bitlocker key from the previous owner, you will need to wipe the disk and reinstall windows.
To do that:
1 download the media creation tool from Microsoft and make it create an USB setup stick
2 boot from that stick (you'll need to use instructions here: https://support.microsoft.com/en-us/surface/boot-surface-from-a-usb-device-fe7a7323-8d1d-823d-be17-9aec89c4f9f5
3 when setup has booted and you are at the language selection screen, press shift F10 so that a command prompt opens.
4 on that command prompt, launch the following 3 commands one after the other:
sel disk 0
5 now after these commands succeeded to clean the disk, close the command prompt and proceed with setup.
A final gotcha: on a surface book2 (which is the only surface device I ever reinstalled), I witnessed a fun-fact: after reinstalling, no keyboard driver was loaded, so the keyboard did not work. I could complete setup only with an external USB keyboard and had to load drivers through windows update.