I had the same thing happen. I was able to win 1 horse competition and then never enter again. I have no mods. It says I have no horses on the lot even though the horse's profile says "at home." I've restarted the game, repaired the game in Origins, debugged the horse, and tried making sims leave and return—I can't do anything to get the game to recognize there is a horse on the lot. I've tried the community board at the sims home with horse on lot and entering with the horse on the competition lot. Nothing. Playing on Macbook in English and the issue is 100% of the time.
