If a user has a personal Google Account with the same email address as their managed Google Account, then they have a conflicting account.
This can happen if a user created a personal Google Account using the domain name of your company or school. If your organization then signs up for a managed Google Account and tries to add those users to your organization's account, they'll have the same address for their personal and work accounts. Two accounts can't share the same email address.
More on conflicting accountsWhen they occur
Conflicting accounts most commonly occur if users sign up for free (or consumer) Google services before your organization signs up for a managed Google Account.
If you create an account that conflicts with an existing account, the existing user will be asked to rename their account. When the account is renamed, all of the data in the existing account will remain outside of your organization’s management.
To learn more about managing conflicting accounts, see Add users who have existing Google Accounts.
Personal Google Accounts that don't use your organization's Google address as the primary or alias email address. For example, a personal Gmail account (firstname.lastname@example.org) isn't a conflicting account.
If you don't have any conflicting accounts, see Options for adding users.
How can I avoid creating conflicting accounts?
To avoid creating conflicting accounts, use the Transfer tool for unmanaged users. For details, see Find and manage existing Google Accounts.