Do you have the audio cues set into the level? Because I think it’s trying to get the volume to adjust the spawn node not the actual audio component which I believe why you get the error. So the way I see that could fix it is to have the adjust audio node connect to a reference of the audio cue you want to adjust.
I can think of 2 ways to do it, one is to have the audio cue placed into the level but disable auto activate so it’s not played twice and then add the reference that way. Or, two, in your BP you can add a variable and change it to be an audio reference and in the settings of that variable, select the audio you want it to reference and compile it, that should work as well.
If it doesn’t work or you don’t understand, I’ll test it out myself and do some screen shots to try and help.