I found a better solution. The validate in_array expects the array to be one of
the parameters in the request. The in: answer will not work if you have 
commas in the array. To use the in_array without having to create a new rule 
you can simply do: ( Make sure you include the .* at the end )

$this->allslots = array('10:00:00', '10:10:00', '10:20:00', '10:30:00', '10:40:00', '10:50:00', '11:00:00', '11:10:00', '11:20:00', '11:30:00', '11:40:00', '11:50:00', '12:00:00', '12:10:00', '12:20:00', '12:30:00', '12:40:00', '12:50:00', '13:00:00', '13:10:00', '13:20:00', '13:30:00', '13:40:00', '13:50:00', '14:00:00', '14:10:00', '14:20:00', '14:30:00', '14:40:00', '14:50:00', '15:00:00', '15:10:00', '15:20:00', '15:30:00', '15:40:00', '15:50:00', '16:00:00', '16:10:00', '16:20:00', '16:30:00', '16:40:00', '16:50:00');

$request['allslots'] = $this->allslots;

validate($request, [
  'field' => 'required|in_array:allslots.*',
]);