If you have this problem most likely means that a Loyverse account with your employee's email address already exists; or that an employee with such email already exists in your account or in a different Loyverse account.
Simply to say, make sure that the email address you are trying to use doesn't belong to another Loyverse account or employee.
In case that there is a Loyverse account with such email, you need to first delete it. You can't do it yourself you need to contact Loyverse team to help you out.
If there is an employee with such email, you also need to delete it first (or at least remove his email from the employee card and his access rights to the back office) - perhaps you created him in a previous Loyverse account, try to figure out.