Please give it a try by adding a validate/validator for sentTo field

sentTo: {
  type: [mongoose.Schema.Types.ObjectId],
  type: [mongoose.Schema.Types.ObjectId],
  ref: "Users",
  ref: 'Users',
  required: [true, "At Least One senderId required"],
  required: [true, 'At Least One senderId required'],
  validate: {
    validator: function (v) {
      return v.length > 0;
    },
    message: 'The sentTo array cannot be empty',
  },