how to fix "mp_dynamicpricing set to 1 but couldn't download the price list"?

[Viper2026]

I keep getting the error "mp_dynamicpricing set to 1 but couldn't download the price list" when I start up my server, but can't find any solution to this problem. Do I need to forward additional ports or something? I've tried adding tcp and udp port 80 as some people said, but that doesn't seem to have fixed the problem.