unable to download safari extension for bitdefender

You don't need anti-virus programs on your computer, so in my opinion, you should uninstall BitDefender. As there are no viruses currently circulating in the wild that can do any damage to OS X, anti-virus programs, at this time, are unnecessary. The majority of them, from what I and other contributors here have seen, negatively affect the performance of whatever system they're installed on. What's more, they don't protect you from what Mac users really need to worry about, which is malware and adware. Malware and adware are installed by you, unlike viruses, which install themselves. The most common way this happens is when you download something via torrent or from an aggregate download site, as many times those installers are bundled with the malware and adware. When you install one, you install all of it.

Please follow the directions from the developer's site to uninstall BitDefender.

Your Mac has all the security needed > About System Integrity Protection on your Mac

Anti virus software will slow your Mac to a crawl.

Hey there. If the blue "Install Now" buttons aren't visible, try using https://extensions.apple.com/ to access Safari Extensions instead of https://safari-extensions.apple.com/. Apple have two web pages for Safari Extensions. One works and the other doesn't. 😉