There's several things you might need to do. Firstly, I'd suggest running the Bitdefender removal tool:
https://www.bitdefender.co.uk/uninstall/Then reboot.
If Windows Security still doesn't activate, you might need to enable it in Settings > Updates and Security.
I've been using it on my new Desktop for about a year, and haven't encountered any issues. The only slight annoyance is that if you've not used the machine for a while, Windows Security whinges that it's not been updated, making you perform a manual update, rather than just getting on with it. Same with scans.