Failed Executing Romance can only happen if the rabbithole found was null, meaning a rabbithole could not be found. This can only happen if you did something, like, say, not installing them (you said you did), not installing at least one tourable rabbithole that is typically used, installing them incorrectly on residential lots where they can't be visited, installing them in tombs, or if, for some reason, the Emperor of Evil happens to be blockading the lot you put all the rabbitholes on with his aura of evil. Sims vulnerable to the aura of evil will not be sent to an object under its influence, so if the Emperor of Evil is blocking all the rabbitholes this way, that particular sim will be unable to find a suitable place to go, as he will be hiding in fear of the Emperor of Evil.
I was wondering if it had to do with the fact that I increased the population of the neighborhood. Would I get this message if the rabbithole is being used by other sims at that exact moment?
This is the first time I've seen this error, and I haven't changed any rabbithole from its original position or deleted any of them, the only thing that really changed is the population going from around 20 yound adults to 34 young adults.