You can check whether the feedback for the specific ticket exists or not using PnP js and if yes, for instance, redirect a user to a Form set with the error message.
Add the code to the Custom Routing section:
var web = new Web('Site URL');
const queryString = window.location.search;
const urlParams = new URLSearchParams(queryString);
const ticket = urlParams.get('Ticket');
if(ticket){
return web.lists.getByTitle('List Name').items.select("Title").filter("Title eq '" + ticket + "'").get().then(function(item) {
if(item.length > 0) {
return 'c46596f4-3325-43d5-86f9-77fc72d52ad4'
}
});