$conn=mysqli_connect('localhost:8080', 'root', ' ','user_accounts' );
$sql="INSERT INTO users
    (name,Lastname,Email_ID,Phonenumber,Password)
    VALUES($firstname,$lastname,$email,$phonenumber,$password)";


if($is_query_run=mysqli_query($conn,$sql))
{
    echo 'Query successfully executed';
    //$query_execute=mysqli_fetch_assoc($is_query_run)
}

Why am i unable to insert data into the database?

You missed the quotes

INSERT INTO users
(name,Lastname,Email_ID,Phonenumber,Password)
VALUES('$firstname','$lastname','$email','$phonenumber','$password')

Also what you are doing is dangerous, it is better to use PDO with prepared statements instead of mysqli_connect