我不知道我是否很好地理解你想要什么，但我按照以下逻辑做了：

• 通过点击按钮提交表单，输入将被验证。如果输入无效，setCustomValidity中指定的消息

函数将出现，如果它是肯定的，该表单将被发送
在脚本执行过程中，代码中的许多问题都是由于无法读取为获取form和input而创建的变量。
此外，您的脚本不包含在验证为false时停止事件的方法。

<form id="myform" class="needs-validation" novalidate>
   <div class="form-group">
     <label for="myInput" class="control-label">Input:</label>
     <!-- The old input did not have its type definition and was not defined as required -->
     <!--<input class="form-control" id="myInput">-->
     <input type="text" class="form-control" id="myInput" required>
     <span id="msg" class="invalid-feedback"></span>
   </div>
   <div class="col-12">
     <button id="btnSubmit" class="btn btn-primary" type="submit">Submit form</button>
   </div>
 </form>

 <script>
  //Setting the custom validity message to "Error" and then displaying the message.
   $(function() {
     //Setting up an event handler for the click event on the button.
     $("#btnSubmit").on("click", function(e) {
       //Getting the first element of the jQuery object.
       var form = $("#myform").get()[0];
       //Checking the validity of the form.
       var isValid = form.checkValidity();
       //Getting the first element of the jQuery object.
       var input = $("#myInput")[0];
       //Preventing the form from being submitted.
       if (!isValid) {
         e.preventDefault();
         e.stopPropagation();
       }
       //Adding the class `was-validated` to the form.
       form.classList.add('was-validated');
       //Setting the custom validity message to "Error" and then displaying the message.
       input.setCustomValidity('Error');
       //Setting the innerHTML of the element with the id of msg to the validationMessage of the input element.
       document.getElementById('msg').innerHTML = input.validationMessage;

       return false; // For testing only to stay on this page
     });
   });

 </script>