Send Array from JS to php file then insert it in wp database through a plugin

This's my code it's success but when but I can't the return data from ajax in the same page. the mechanism is the user click the dropbox button and when he selects it returns dropbox_links and then insert dropbox_links in database using add_post_meta

The Insert should be on the spot becuase the dropbox button click is the only trigger

 $.ajax({
    
                type: "POST",
                data: {
                    req: JSON.stringify(dropbox_links)
                },
    
                cache: false,
                success: function(responseData) {
                    // consider using console.log for these kind of things.
                    console.log("Data recived: " + responseData);
                    console.log(dropbox_links);
    
                },
                error: function(msg) {
                    console.log("not sure what to ask for here to check issue" + msg);
                }
            });

This's the php code but I don't catch or print any data so should I re-run the whole php script to see if it returns any data?

function printdata()
{
    $data    = $_POST["req"];
    $data    = json_decode($data, true);
    print_r($data);
}

if (isset($POST["req"])) {

    printdata();
} else {;
}