Der Vaterland is the most challenging marketplace in the game, so don't feel bad if you're having trouble here! The competition at Der Vaterland changes every season based on what Germans typically drink during that time of year, so you need to be quick and precise with your beer stocking to get it right.
If your batch Quality isn't around 1300 or better, sell elsewhere and level up your Employees and Equipment until your Quality improves.
Once you hit the 1300 point, one sure-fire way to succeed at Der Vaterland is to wait for the Winter season. It's the longest lasting of the various seasons there so you have more chance to sell with success.
The first time the winter season comes around, do the research for what type of beer typically is wanted there, and then brew and keep a few batches in storage until Winter hits.
Then, sell them one at a time, in order of highest to lowest Quality, shooting for around 600 or 700 coins profit per batch.
Do this for 4 or 5 years and you should reach the $10K profit needed to complete the challenge. Prost! ?