Looks like the cloud-router-routing-protocols is requiring direct_equinix_ipv6_ip to be provided, even though the variable is set up to default to "".
This is a stumbling block for users using other modules like cloud-router-2-google-connection, which does not configure any sort of IPV6 on the google interconnection and thus there is no IPV6 value to pass in here.
We can continue to require users to pass in "" manually, but it'd be a smoother experience if the cloud-router-routing-protocols module just didn't require direct_equinix_ipv6_ip to be passed in at all.