I wouldn't worry about you not advancing because of this, there are other methods; I personally prefer mental vibration, it's easier for me and doesn't catch the ear of my family plus, it can be more effective if used properly. I also endure this problem, having never learned how to roll my R's stopped me from using vibrations for a while, so I was late to open my chakras (but also doing other meditations of course), until of course I found out you could do mental vibrations.
Well most can't roll their r's simply because they can't curl their tongues, if you can then you most likely can learn to roll them. If you can't I guess you can try to make a deep heavy vibration from the pit of your throat to simulate rolling your r's.