How to show custom error message in jQuery validate below bootsrap select picker?

Usually, you can change position of error message in jQuery validate using below jQuery code

1. If you want to specify location of 1 or 2 fields and keeping same for other form elements, you can use jquery validate like code

$("#Form1").validate({
errorPlacement: function(error, element) {
    //for name attribute
    if (element.attr("name") == "email" )
    {
        error.insertAfter(".some-class");
    }
    else if  (element.attr("name") == "phone" )
    {
        error.insertAfter(".some-other-class");
    }
    //for rest of the elements, keeping same position
    else
    {
        error.insertAfter(element);
    }
}
});

2. For you above select picker, you can try this modified code to place error message below bootstrap selectpicker

$("#Form1").validate({
                errorPlacement: function (error, element) {
 
                    //check if element has class "kt_selectpicker"
                    if (element.attr("class").indexOf("kt_selectpicker") != -1) {
                       //get main div
                        var mpar = $(element).closest("div.bootstrap-select");
                        //insert after .dropdown-toggle div
                        error.insertAfter($('.dropdown-toggle', mpar));                       
                        
                    } else {
                     //for rest of the elements, show error in same way.
                        error.insertAfter(element);
                    }
                }
                   
                
            });

3. For Showing custom error message, use message attribute

$("#registration").validate({
    // Specify validation rules
    rules: {
      firstname: "required",
      lastname: "required",
      email: {
        required: true,
        email: true
      },      
      phone: {
        required: true,
        digits: true,
        minlength: 10,
        maxlength: 10,
      },
      password: {
        required: true,
        minlength: 5,
      }
    },

    //specify custom messages
    messages: {
      firstname: {
      required: "Please enter first name",
     },      
     lastname: {
      required: "Please enter last name",
     },     
     phone: {
      required: "Please enter phone number",
      digits: "Please enter valid phone number",
      minlength: "Phone number field accept only 10 digits",
      maxlength: "Phone number field accept only 10 digits",
     },     
     email: {
      required: "Please enter email address",
      email: "Please enter a valid email address.",
     },
    },

         // specify error placement
      errorPlacement: function (error, element) {

                    if (element.attr("class").indexOf("kt_selectpicker") != -1) {
                        // $(".dropdown-toggle").text(error);
                        var mpar = $(element).closest("div.bootstrap-select");
                        error.insertAfter($('.dropdown-toggle', mpar));                       
                        
                    } else {
                        error.insertAfter(element);
                    }
                }
  
  });