Why the $this->find( $id ) is generating error but get()->getRow() is working?

Check if column that stores primary key in your table is named "id" and indexed as primary key. Otherwise (e.g. name is "some_id") you should define it in your model:

<?php namespace App\Models;
use CodeIgniter\Model;
class UserModel extends Model
{
protected $table = 'users';
protected $primaryKey = 'some_id';
?>