Avast Free Zone cannot open link

When trying to open Finnish Broadcast Company’s net radio, I get a message: Avast Free Zone cannot open link because application added to protocol ‘rtsp’ was not found from the system.

What to do?

I’m not sure what ‘Avast Free Zone’ is? ???

RTSP = Real Time Streaming Protocol

Just a guess, but I think the OP means the SafeZone Browser.
https://www.avast.com/faq.php?article=AVKB209

The SafeZone browser is (to my information) based on Opera.
That means a plug-in is needed in order to be using the RTSP.