Laravel validation checkbox

Use the accepted rule.

The field under validation must be yes, on, 1, or true. This is useful for validating “Terms of Service” acceptance.

Sample for your case:

 return Validator::make($data, [
    'firstName' => 'required|max:255',
    'lastName' => 'required|max:255',
    'email' => 'required|email|max:255|unique:users',
    'password' => 'required|confirmed|min:6',
    'checkbox' =>'accepted'
]);