Why can't I change ringtones on a certain contact on my phone, it's at Windows 10 10581.0 on my Lumia 535?

[Windows Central Question]

Why can't I change ringtones on a certain contact on my phone? It's at windows 10 10581.0 lumia 535

Every time I try to change a contact's text tone or ringtone it says that I need an APP to do the task but when I press YES there's no such app at the app store.
Please help. thank you.