What could be causing the database retrieval error?

I'm using CodeIgniter 4, and I encountered an error (Call to a member function table() on null) when trying to retrieve records from the database using the following code:
$CI = & get_instance();
$CI->db->select('*');
$CI->db->from($table_name).
I then referred to the documentation and tried using the $db->table("tablename") method, but it also failed.