Error 'Portal account owner must have a role' when you enable an External User

This error is generated because the user who running the code does not have a role.

Ensure that the user running the code below has a role assigned (eg. Sys Admin user does not have a role assigned by default). To resolve the error, assign the User a Role. 

You will also see this same error (for the same reason) when trying to enable a contact as a partner or customer portal user.

The following sample code demonstrates how to avoid this error in a Test class:

UserRole userrole = [Select Id, DeveloperName From UserRole Where DeveloperName = 'CEO' Limit 1];

User adminUser = [Select Id, UserRoleId From User Where Profile.Name='System Administrator' Limit 1];

adminUser.UserRoleId = userRole.Id;
update adminUser;

System.runAs(adminUser){
    Account a = new Account(Name='Test Account Name');
    insert a;

    Contact c = new Contact(LastName = 'Contact Last Name', AccountId = a.id);
    insert c;

    User user = new User();
    user.ProfileID = [Select Id From Profile Where Name='Customer Community User'].id;
    user.EmailEncodingKey = 'ISO-8859-1';
    user.LanguageLocaleKey = 'en_US';
    user.TimeZoneSidKey = 'America/New_York';
    user.LocaleSidKey = 'en_US';
    user.FirstName = 'first';
    user.LastName = 'last';
    user.Username = 'test@uniquedomain.com';
    user.CommunityNickname = 'testUser123';
    user.Alias = 't1';
    user.Email = 'no@email.com';
    user.IsActive = true;
    user.ContactId = c.Id;

    insert user;
}