How to implement Server Side DataTable in CI4

I expect your index route ('/product-codes(:any)') is matching your AJAX call. Try switching the order of the routes, or making the first route more specific.

$routes->add('/product-codes(:any)', 'Code8ProductCode::index'); //This is the Page Itself

$routes->add('/product-code/showProductCodes(:any)', 'Code8ProductCode::showProductCodes'); //This is the Ajax Request 

the 2nd one is the Ajax request call