Yes, a sling strap can work with a backpack, and some photographers use that combination without major problems. In general, you would wear the sling over your shoulder/body rather than trying to route it under the pack straps.

That said, comfort and practicality depend on your setup. Several common downsides were mentioned:

• the camera can hang at your hip and bump into your leg, belt, jeans buttons, or the backpack
• there may be only a small “sweet spot” where it hangs comfortably
• the camera can wobble while walking and gets worse if you run
• accessing or removing the backpack becomes more awkward because the sling is in the way

For hiking or active movement, many people find a neck/shoulder carry or storing the camera in the pack better when not actively shooting. If you do use a sling with a backpack, expect to steady the camera with a hand at times.

So: usable, yes; ideal for everyone, no. It’s most comfortable for slower walking and quick camera access, but less convenient for running or frequent backpack access.