I just ran into this same issue after having used the visa infinite contract number to book a one-day rental in TPA. The rep confirm that there is a comment on her screen where rental credits would normally appear that said 'org. does not support emerald club', which apparently means they opted out of rental credit accumulation as a cost saving measure for their contract. She gave me 5 rental credits (to push me to a free day) for my trouble.

In the future I will be using either my company's contract number or the base emerald club member contract, 5007125. The latter looks to have rates similar to the visa infinite contract anyway and also allows free additional drivers.