So from that page I go Ringtone> Plus Sign (top right) and it takes me to the original page (screenshot) with that error.
I have restarted phone and made sure I have no outstanding updates.
The app you selected to make custom ringtone needs storage permisson. For me it was sound picker. To give it permisson go to Settings-Privacy-Permisson Manager-Storage, then show system by using 3 dots on top right. Now scroll down to sound picker and allow.
I am having this same isssue as the original poster, phone has been updated and restarted, checked the permissions for sound picker and it's definately enabled. Still takes me back to the same place as the screenshot that OP posted. It seems like the blue "settings" on the notification it gives you would be clickable but it is not, so it doesn't redirect you to any settings, it just tells you that info and nothing else.
Make sure that the file you are trying to access is in phone memory and not on an sd card. Android limits access to sd cards for 'security' reasons and that me be causing a conflict.
I have the same issue ,the ringtone was downloaded using email attachment. I can play it no issue but unable to set it as ring tone.I tried to move to different folders in phone memory no use.I do not have a sd card .
Thanks in advance
I had the same problem on my S9, and I've just figured it out. Here is how to sort it.
1. Go into settings and click on apps.
2. When you see the list click on the 3 vertical dots in the top right corner.
3. Click on "permission manager"
4. Scroll down and click on "storage."
5. Click on the 3 vertical dots again and click on "Show system", give it time to load all the system apps, it may take a while.
6. Scroll down and click on "Sound Picker", then "Allow" storage access for this App.
7. That's it. The + button will now load your tracks.
I hope.this works for you.