Rest assured, you're safe to post the video!
Facebook uses automated algorithms to detect whether music sounds like other music, but they're often wrong. Our media library consists only music from the creative commons and the Youtube public music library, which have been pre-vetted by Google to ensure copyright-free status.
You can simply choose the option to continue with posting.
If you're curious to learn more about how and why these false positives happen, here's an example:
This is "Not As It Seems", a copyright-free track from Kevin MacLeod:
And here is "Better Repent", a copyrighted track from Sound Cloup:
You can hear in the above example that "Better Repent" uses a small part of "Not As It Seems" which is a copyright-free track as a mashup for their intro, but beyond that nothing sounds the same.
Auto-detection algorithms from social networks like Facebook and Instagram picks up on the similarities and asks you for confirmation that the track is copyright-free. This is why the false positives happen, and also why it's fully safe to proceed with posting it.