أحسن الله إليكم If I am about to pray salatul Tawbah and one of my parents ask for a small favour, which should I do first?

I'm not sure if you suffer from waswas or not, but if you do, I would like to point your attention to the fact that repentance can happen at any time. You can seek forgiveness and repent to Allah without praying two rak'ahs. If your waswas leads you to making it a ritual where you feel you can never repent unless you pray two rak'ahs, then this becomes a bid'ah, and this is not allowed. Coming back to your question: take care of what your parents need first, and then you can pray two rak'ahs.