Call to a member function fetch_object() on a non-object and undefined variable: res

You have to check if the query was successful. See PHP: mysqli_result::fetch_object - Manual[^].

The second error informs you that the $res variable has not been defined when used in the return statement. It has been defined in the loop but is out of scope (not seen outside the loop). So you have to define it first in the scope of the function.

So use something like (untested):

public function sel($con,$t1)
{
    $res = array();
    $sql="select * from $t1";
    $q=$con->query($sql);
    if ($q)
    {
        while ($fet=$q->fetch_object())
        {
            $res[]=$fet;
        }
    }
    return $res;
}

Note also that $con must be a valid connection (check it before calling the function).