Hey there kateryna296,
We understand you are trying to set a song for your alarm, but it defaults to the Radar sound. We'd be happy to provide information to help you troubleshoot this.
To get started, if you're using a song from Apple Music, we'd recommend ensuring that the song is downloaded to your device using the steps found here under "Download music from Apple Music to your iPhone": Add music to iPhone and listen offline - Apple Support
If after downloading the song, you're still unable to use it as an alarm, we'd suggest testing to see if the same behavior occurs when changing the alarm to a different default alarm option. For example, if you're currently using Radar, let's try Apex, Beacon, etc. Does the same behavior occur? If so, if you create a new alarm are you able to use a different option? This can help us isolate the behavior further to the app itself, songs specifically, or a particular alarm.
Let us know, and we'll go from there.
Regards.