Check your domain’s settings. You likely need to increase the domain quota.
In your “Domains” tab you’ll see a “Quota” column that shows how much you’ve allocated vs. maximum.
For example, if your domain is set with a Quota of 10GB and you create 2 years with 4GB each, you’ve now used 8GB of your quota.
When you try to add another user with 4GB (or something more than 2 in this case), you’ll exceed the quota configured for the domain and cannot proceed.