Well, that’s your problem. sub?id is what defines which uids and gids are available to a user for purposes of making user namespaces. It’s strange that those files don’t already exist; useradd should create them automatically. What distro are you using?
Regardless, you can create those files yourself. Here’s a line from subuid my machine: administrator:100000:65536. The first field is the username (you can also use a uid), the second is the starting uid for the block of uids, and the third field is the number of uids in that block. So uids from 100000-165535 (inclusive) are allocated to the user administrator.