I am still very confused here

Tell us what’s happening:
Describe your issue in detail here.

   **Your code so far**
/* file: index.html */
<!DOCTYPE html>
<html>
 <head>
   <title>Registration Form</title>
	  <link rel="stylesheet" type="text/css" href="styles.css" />
 </head>
 <body>
   <h1>Registration Form</h1>
   <p>Please fill out this form with the required information</p>
   <form action='https://register-demo.freecodecamp.org'>
     <fieldset>
       <label>Enter Your First Name: <input type="text" name="first-name" required /></label>
       <label>Enter Your Last Name: <input type="text" name="last-name" required /></label>
       <label>Enter Your Email: <input type="email" name="email" required /></label>
       <label>Create a New Password: <input type="password" name="password" pattern="[a-z0-5]{8,}" required /></label>
     </fieldset>
     <fieldset>
       <label><input type="radio" name="account-type" /> Personal Account</label>
       <label><input type="radio" name="account-type" /> Business Account</label>
       <label>
         <input type="checkbox" name="terms" required /> I accept the <a href="https://www.freecodecamp.org/news/terms-of-service/">terms and conditions</a>
			  </label>
     </fieldset>
     <fieldset class=":last-of-type">
       <label>Upload a profile picture: <input type="file" name="file" /></label>
       <label>Input your age (years): <input type="number" name="age" min="13" max="120" />
			  </label>
       <label>How did you hear about us?
         <select name="referrer">
           <option value="">(select one)</option>
           <option value="1">freeCodeCamp News</option>
           <option value="2">freeCodeCamp YouTube Channel</option>
           <option value="3">freeCodeCamp Forum</option>
           <option value="4">Other</option>
         </select>
       </label>
       <label>Provide a bio:
         <textarea name="bio" rows="3" cols="30" placeholder="I like coding on the beach..."></textarea>
			  </label>
     </fieldset>
     <input type="submit" value="Submit" />
   </form>
 </body>
</html>
/* file: styles.css */
body {
 width: 100%;
 height: 100vh;
 margin: 0;
 background-color: #1b1b32;
	color: #f5f6f7;
 font-family: Tahoma;
	font-size: 16px;
}

h1, p {
 margin: 1em auto;
 text-align: center;
}

form {
 width: 60vw;
	max-width: 500px;
	min-width: 300px;
	margin: 0 auto;
}

fieldset {
 border: none;
	padding: 2rem 0;
}

.fieldset :not(:last-of-type){
 border-bottom: 3px;
 solid: #3b3b4f
}

label {
	display: block;
	margin: 0.5rem 0;
}

   **Your browser information:**

User Agent is: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/102.0.5005.63 Safari/537.36

Challenge: Step 41

Link to the challenge:

use fieldset:not(:last-of-type) {}