Why won't Bitdefender work with spybot and does Bitdefender do what spybot does??

Spybot Search and Destroy is not what it once was...; 6-8 years after it's brief flash in popularity, it seems to find mostly lots of cookies in the tests I've seen run.

I'd suspect BItDefender to be more competent and working with a more frequently updated detection engine/signature database....
 
If they both have a real-time antivirus component, one of them needs to be disabled or uninstalled, since it's inadvisable to have two or more AV scanners installed at the same time.

https://www.safer-networking.org/faq/live-protection-is-conflicting-with-my-other-av-program/

In all honesty I would ditch Spybot -- it seems to have lost the plot several years ago.