$(function () {
	$("#error_number").hide();
	var error_number = false;
	$("#form_number").blur(function(){
		check_number();
		console.log("dsa")
	});
	function check_number() {
		var number_length = $("#form_number").val().length;
		 
		if (number_length != 11) {
			console.log("hello")
			$("#error_number").html("the number must be 11 digit");
			$("#error_number").show();
			$("#form_number").css("border-bottom","2px solid #F90A0A");
			error_nubmer = true;
		} else {
			console.log("hell")
			$("#error_number").hide();
			$("#form_number").css("border-bottom","2px solid #34F458");
			}
		}

	$("#myForm").submit(function() {
	  error_number = false;

	  check_number();
	 
	  if (error_fname === false ) {
	    alert("Sent Successfull");
	    return true;
	   } else {
	    alert("Please Fill the form Correctly");
	    return false;
	  }
	});
});

<div class="form-group" >
  <label for="exampleInputnumber2">Mobile     number</label>
  <input minlength="11" maxlength="11" type="text" class="form-control" id="exampleInputEmail2" id="form_number" placeholder="01234567890" required="required">
  <span class="the-message">Please fill this feild</span> <span id="error_number"> Please enter a correct phone number</span>
</div>