It is more than a half year ago but I can show you the answer and maybe it is important for other people too.

The error message comes from the fact that the invoice currently only allows 19% VAT and the product you want to sell has 16% VAT. To fix this error you have to go to your Accounting Application (Finanzen) and then under Configuration (Konfiguration) you have to click on "Chart of Accounts" (Kontenplan). Now you can search for the code number "8400" which is called something like "Erlöse 19 % USt". Here you need to click on "Establishment" (Einrichtung) on the right side of the line. After clicking on "Edit" (Bearbeiten), you can select "Standard taxes" under the point account options and you have to search for "16% Umst" by using the arrow on the right side of the line. Now you have the two options to sell products in this special case with 19% VAT or 16% VAT. Don't forget to save your new selection ; )