I want to make a bone broth but the butcher didn’t have any beef bones, but did have some lamb bones - can they be subbed in

I like lamb OK (although I find lamb fat particularly repulsive), but I'd say no to the substitution, it would be a radical difference in flavor.

I'd use beef rib bones before I'd use lamb...I agree with Stephanie B lamb is way too gamy for my taste:
I've been using chicken pieces with the bones on to make broth. I heat little saucers of the broth for my little dogs and they love it!

Sure you can make a broth from lamb bones, but I hope you like sheepy flavors because it will have that distinctive taste. It will be a strong tasting broth. I wouldn't make that substitution, but that is purely based on my dislike of sheep/lamb products.