E-mail templates need a principal model in order to be able to show certain details inside said templates. For example, the picture of the vehicle, characteristics like amount of suitcases or doors and deposits/guarantees/franchises.

If the category doesn't have a principal model assigned, it can't generate the e-mail because there is information it won't be able to fill in.

In order to use the template normally, make sure you assign a principal model to the category.