Ultimately natural selection must operate on the gene. Genes are the only source of information that gets passed to offspring through germ line cells in sexual reproduction or mitosis in asexual reproduction (don’t get me started on the fad of epigenetics, which is just a fancy term for standard DNA controlled embryological differentiation.)
The replication of genes and the information they encode, are the physical cause of the effect of phenotypes.
Group selection theorists (of which there are few) have no physical cause that allows selection to occur on the level of the group, and there’s no sound hypothesis of such that I have heard of. You’d need some physical mechanism for information flow between individuals in a group for that to be the case, and outside of kin inheritance, there’s nothing like that that exists.