@aodoherty Ok. I went into my game and took a look at a number of my households. In all cases but one, all the sims could do the serenade option on both piano and digital keyboard. Btw, if it works on one, it works on both. If it doesn't work on one, it works on neither so the issue isn't with the piano or digital keyboard per se. In one case, when I tried, the sim and his romantic partner were on different floors, the option was greyed out, but after getting both sims on the same floor level the option went active. I also tried on pipe organs but those don't appear to have the serenade option at all.
In the one case I couldn't get the option to come up for a sim with a romantic partner, was a special case and really doesn't apply to you since all your sims are YAs. I thought it might be an age difference thing then but after looking at your simsinfo pics again, it's not since they're all YAs. I use mods that allow certain relationships that the unmodded game doesn't allow. But what I found in that one case was the serenade option shows up as greyed out on both sims and neither sim has the serenade option active. It appears to ignore the romantic relationship altogether which I would expect in that case since it's one the game doesn't allow normally. Further checking showed that all relationship types (YA & Elder, YA & Adult, etc.) do get the option. I also checked my vampires and spellcasters and they still do as well. Didn't look at aliens since none of you sims are aliens.
So what does that leave to try? I am guessing now and the only other thing I can come up with is the possibility of CC interference so if you have any CC pianos or digital keyboards you might try removing those. I have a couple of cc pianos but the option did show up on them but it may not on others.
You may have to try recovering and using an older save as @RandomBuzziness suggests. Or you could try saving the sims one at the time to your library, deleting them and bringing them back in and recreating the relationships again . I wish I could help you further but I've run out of things to suggest.
