terraform-provider-sops | A Terraform provider for reading Mozilla sops files | Infrastructure Automation library

I think the "set of map of string" part of this error message is the important part: for_each requires either a map directly (in which case the map keys become the instance identifiers) or a set of individual strings (in which case those strings become the instance identifiers).

Your example YAML file shows yaml_users being defined as a YAML sequence of maps, which corresponds to a tuple of objects on conversion with yamldecode.

To use that data structure with for_each you'll need to first project it into a map whose keys will serve as the unique identifier for each instance of the resource. Assuming that the name values are suitably unique, you could project it so that those values are the keys: