Try for particular user (case keystone api v2.0) :-

# keystone user-role-add --tenant_id=$UUID_PROJECT_TENANT \
  --user=$UUID_PARTICULAR_USER --role=$UUID_ResellerAdmin_ROLE

Anyway try to add ResellerAdmin role to user .

I would not recommend ResellerAdmin role for users because that allows that user to access/modify anything/everything in Swift.

When you created the user, what roles did they have on the project? Then look at /etc/swift/proxy-server.conf -- look for operator_roles in the keystoneauth section. Your user needs one of these roles to create a container.

At a guess, when you created the user, the user has _member_ role for the project. A common pattern is to modify operator_roles by adding _member_ to the list then restart proxy-server. Your user should now be able to create containers and objects.