There's nothing there / Page not found error
The padlet URL you have tried to access can either be one of 2 things:
1. URL error. Check again with the padlet creator if the URL shared is correct. If there's no error, there's a chance the creator may have also updated the address from something like padlet.com/mrsmith/r43r6geFD to padlet.com/mrsmith/activity12.
2. Padlet is deleted. There's not much we can do in this case, but it is likely that the padlet owner has created a new one accessible to everyone