Hy D R,
for me it seems to be clear. As your batch number definition seems to be to long (> 20), the batch number can not be stored and you get the error meassge.
Your process seems to defined to create the batch number at purchase order activation and the definition of the batch number exceeds the data type lenght of the batch number, which is 20 characters defined by that ETD.
So please go to your master data of the item.
Expand "Manage Inventory" fast tab. Under "Tracking" you see "Batch number group".
Go to the Batch number group master data and make a screen shot and share this here.
At my demo it looks like this:

I've activation at puchase order and I use a number sequece for Batch number creation which is shorter than 20 characters.