Information about the objectives in this puzzle. (All objectives give +1000 points when fully satisfied, for a maximum of +10 000.)
Most objectives have a "Show" button which should highlight areas of concern in red. (Even when fully satisfied, objectives may highlight groups which may eventually contribute.)
The program is allowing the formation of a sulfonium ion similar to the ammonium ion previously reported.
On the thiazole, substitute the sulfur with nitrogen to form an imidazole. Then substitute this nitrogen back to sulfur again. You will now have a valid charged sulfonium ion (a protonated sulfur) species. There is no immediate undesired group penalty for doing this as there now is for ammonium ion formation. The hydrogen can be selected, but not deleted. It can also be replaced with carbon to methylate the sulfur. This does give an undesired group penalty.