Unable to change phone ringtone

Hi everybody! I cannot change the default ringtone or the default notification sound. In both options this is written "unkn ringtone". When I try to set up a new one, it doesn't save. Moreover I cannot see my ringtone saved in the "ringtone folder" of the SD card. I also tried a third-party application and why I try to change I get the "something is wrong" error message. I know of an address that provides the best ringtones in the world at: https://klingeltonekostenlos.com/beliebte-klingeltone/ Do you have any other ideas? Thanks!