Programmatically validate a user
The error message doesn't specify which field caused the validation error. In this case, the error is caused by setting the init field with a value that isn't a valid email address.
Differently from Drupal 7, in Drupal 8 the init field for the User entity is created using the email datatype. (See User::baseFieldDefinitions().)
$fields['init'] = BaseFieldDefinition::create('email')
  -&gt;setLabel(t('Initial email'))
  -&gt;setDescription(t('The email address used for initial account creation.'))
  -&gt;setDefaultValue('');

Differently from the mail field, it doesn't get further constrains. (Compare the code used to create the init field with the following code.)
$fields['mail'] = BaseFieldDefinition::create('email')
  -&gt;setLabel(t('Email'))
  -&gt;setDescription(t('The email of this user.'))
  -&gt;setDefaultValue('')
  -&gt;addConstraint('UserMailUnique')
  -&gt;addConstraint('UserMailRequired')
  -&gt;addConstraint('ProtectedUserField');

Still, as any entity field whose datatype is email, it has a constrain defined in the annotation of that datatype.
@DataType(
   id = &quot;email&quot;,
   label = @Translation(&quot;Email&quot;),
   constraints = {&quot;Email&quot; = {}}
)

(The classes defining that constraint are EmailConstraint and EmailValidator.)
Setting the init field for the User entity is optional, but it's generally used from administrators of a site when users claim they created an account for which they cannot retrieve the password they lost.
Depending on the purpose of the code you are using, you can either set the init field, or not. If you set it, it must set to a valid email value.