In no particular orger
1. Lack of Interservice Cooperation between Heer, Kriegsmarine, and Luftwaffe
2. Lack of coherent planning, replete with errors.
3. The invasion is to be carried out in barges, designed for travel along the Rhine River. There are too few to transport the required invasion, much less the heavy equipment, and even mild waves (or a passing destroyer) can capsize them. Said invasion barges are also horribly undermanned.
4. The absurd superiority of the Royal Navy, whose forces within the channel can wreak havoc among the invasion barges and whose forces from Scapa Flow can easily wipe out the Kriegsmarine should it interfere
5. The absurd number of tasks the Luftwaffe must carry out in order for the invasion to be successful, especially in the face of the RAF. (and no, continuing to hit the airfields isnt enough to eliminate the RAF).
6. The superiority of British forces available to cover the landing zones, and the ability said british soldiers have to summon decisive reinforcements.
7. The fact that, the one time the Germans tested a landing, under optimal conditions, they lost about 10% of their force (which missed the target beach), and judged it a success.
8. The lack of an ability to resupply landing forces, especially in the event of an opposed landing (resupply would be over open beaches).
9. Terrain is perfectly suited for bottling up the Germans.
10. The Germans lack any reliable means to stop the RN.
I see that Ian and Allison's essays have been linked. use those to back you up. Also, point out that the invasion has been wargamed by Sandhurst, and that every time it failed in a matter of days.