Patingin ng form mo sa html. Yung <form></form> tag. Baka mali or wala kang nalagay na "name" attribute sa input field.
yan paps
<form action="register.php" method="POST">
<div class="form-group input-group">
<div class="input-group-prepend">
<span class="input-group-text"> <i class="fa fa-user"></i> </span>
</div>
<input name="fullname" class="form-control" placeholder="Full name" type="text" required>
</div> <!-- form-group// -->
<div class="form-group input-group">
<div class="input-group-prepend">
<span class="input-group-text"> <i class="fa fa-envelope"></i> </span>
</div>
<input name="email" class="form-control" placeholder="Email address" type="email" required>
</div> <!-- form-group// -->
<div class="form-group input-group">
<div class="input-group-prepend">
<span class="input-group-text"> <i class="fa fa-phone"></i> </span>
</div>
<select name="code" class="custom-select" style="max-width: 120px;" value="" required>
<option selected hidden value="">Area Code</option>
<option value="+63">+63</option>
<option value="+72">+72</option>
<option value="+82">+82</option>
</select>
<input name="phone_number" class="form-control" placeholder="Phone number" type="text" required>
</div> <!-- form-group// -->
<div class="form-group input-group">
<div class="input-group-prepend">
<span class="input-group-text"> <i class="fa fa-building"></i> </span>
</div>
<select name="job" class="form-control" required>
<option selected=""> Select job type</option>
<option>Front End Developer</option>
<option>Full Stack Developer</option>
<option>System Developer</option>
</select>
</div> <!-- form-group end.// -->
<div class="form-group input-group">
<div class="input-group-prepend">
<span class="input-group-text"> <i class="fa fa-lock"></i> </span>
</div>
<input name="password" class="form-control" placeholder="Create password" type="password" required>
</div> <!-- form-group// -->
<div class="form-group input-group">
</div> <!-- form-group// -->
<div class="form-group">
<button name="btn-register" type="submit" class="btn btn-primary btn-block"> Create Account </button>
</div> <!-- form-group// -->
<p class="text-center">Have an account? <a href="">Log In</a> </p>
</form>