[SalesForce] Customize the Duplicate Username error message in a Community

Your user need to be assign a role. So go to User> select your user > Edit and i Role give it any role(eg CEO) Then Save it.

It will solve your problem

I answered that question to which you refer. Here's my answer from other there..

The Guest user assigned to new accounts created by community self-registration has no role. I solved this problem by adding logic to the Insert Before trigger on the Account object that reassigns accounts owned by this user to a "real" user. You may want to come up with a more elegant method that doesn't embed record Id's, but this should work.

trigger AccountTrigger on Account ( before insert ) {
    if (Trigger.isBefore && Trigger.isInsert) 
        AccountTriggerHandler.beforeInsert(Trigger.New);
}

public without sharing class AccountTriggerHandler {

    public static void beforeInsert (List<Account> accounts) {
        system.debug('AccountTriggerHandler.beforeInsert accounts: ' + accounts);
        Id guestId = '0056A00000zzzzzzzz'; // the Id of the Guest User in the community
        Id defaultOwnerId = '0056A00000zzzzzzzz'; // the Id of a "real" user
        for (Account account : accounts) {
            if (account.OwnerId == guestId ) {
                account.OwnerId = defaultOwnerId;
            }
        }
    }
}

I believe the real answer to your quest is to create a multi-step VisualForce wizard that creates the account/contact records in one method, and then creates the user you're looking for in the next step. When you get the second step, you should be able to add that contact to a community because you've set Account.OwnerId to an owner with a role in the Account Trigger.