Hi,Thanks for helping.
No, the only thing that occurs is the (one_page_checkout error log gets bigger) and the bottom of it shows the error I included in last post, not sure if you need to see any other info from that log.
First I disabled the Express Module, error still came up,
Than I disabled Flat Module, error still came up again,
After I disabled BOTH shipping modules and only left "Store Pick Up" as an option but that came up with a Different error now and the following debug log:

Code:
[26-Jul-2021 08:03:12 Australia/Sydney] PHP Notice:  Malformed value for session-based shipping module; customer will need to re-select: false in /home2/...../public_html/....com.au/includes/classes/order.php on line 323
Error snapshot: https://i.imgur.com/313GGoE.png

Weird all this seem to happen when the one_checkout is enabled.

My flat rate file names are (flat.php and express.php), I've checked both files well and they are ok, and they both work fine in normal zen cart checkout process.