The validation errors can be defined in a key-value pair manner but you have to provide the appropriate structure in order for it to work.
Below is an example of how you can define the error validation message.
<?php # resources/lang/en/validation.php return [ 'custom' => [ 'email' => [ 'required' => 'We need to know your email address!', 'max' => 'Your email address is too long!' ], ], ],