From what I've read Rommel's sequel was a project he started much later, during or shortly after the African campaign. He died before he could get very far. He could hardly have written it any earlier than that since prior to 1939 there weren't any proven tank tactics to write about, and Rommel himself didn't participate in developing them or command armored units until 1940.
It is believed he started writing it while commandant of the War Academy at Wiener Neustadt (Theresian Military Academy) in 1938, though he may have started earlier during his time as an instructor at the Potsdam War Academy (1935–37). His career as a military theorist and instructor, however, was preempted by his duties as a commander and soldier during World War II. For the next several years he was too busy as a practitioner of armored warfare to write about it.
Following the end of the North African Campaign in 1943, he again started work on it, drawing upon his, by then, extensive experience and notes. But once more war, along with ill health, interfered as he was put in charge of the defense of Normandy in 1944. After his staff car was strafed by an RCAF Spitfire fighter on July 17 of that year, he again started on the work while recuperating from his injuries. But he was soon implicated in the July 20 Plot against Adolf Hitler and compelled to commit suicide on October 14.
The Tank In Attack exists only in the form of scattered manuscripts and notes, but due to the fame of its author, it has achieved legendary status as a work of military literature. Most of what was to be in The Tank in Attack can be found in the book The Rommel Papers, which is made from notes and diary entries by Field Marshal Rommel during the Africa campaign.