There were plans, but remember that up to March 27, Yugoslavia was a German ally. The timeline:
- March 27, the anti-German coup happens.
- March 29, OKH issues its initial invasion orders.
- April 6, the attack is launched.
- April 17, Yugoslavia unconditionally surrenders.
That's two days to plan the invasion (or at least massively update existing plans), a week to prepare, and then a week and a half to win, with total casualties of just 500 men compared to nearly 400,000 for the Yugoslav army.
The Germans were helped by the fact there were a lot of troops in Eastern Europe already to prepare for Barbarossa. Ironically, the invasion of Yugoslavia served to confirm to Stalin that all those troops were actually not intended to fight Russia, but to clean up the Balkans.