The Allies might have succeeded in capturing a major French port in 1942 if they’d thrown enough resources at it, but: 1) it would have been much more costly in casualties that it was to seize the Normandy beaches in June 1944; 2) it’s doubtful that the Allies could have held it for very long; and 3) they would have had no chance of breaking out of it to liberate France – so the operation would have been pointless. The explanation for each of those problems is the same: in 1942, the Allies simply didn’t have the massive resources that were needed to carry out the Normandy landings and to then drive the Germans out of France. Those resources included trained manpower, the aircraft required to achieve air supremacy over western France (and to degrade the Luftwaffe in the months prior to the invasion), sufficient transport ships and landing craft, and the PLUTO fuel pipeline system. Even with all those resources, it took the Allies a couple of months to crack the German defenses in western France in 1944 and drive the Wehrmacht back towards Germany – at which point the Allied advance itself started running into problems with overstretched logistics (chiefly fuel). So a premature landing in 1942 would basically have turned into a combination of Dieppe in WWII and Gallipoli in WWI.
As a further complication, the British and the Americans in 1942 were at loggerheads over the fundamental question of whether Western France should even be attacked in the first place, and they remained so for nearly two years – so this was hardly an ideal moment to do something as hazardous as trying to seize and hold a major French port, without any viable plan for what to do afterwards. Roosevelt ultimately had to force Churchill to support a Normandy landing by doing an end-run around him: ahead of a three-power meeting with Stalin in (I think) 1943, FDR and his people evaded Churchill’s questions about strategic objectives, saying that the matter would be discussed when the three leaders were in the same room together. Then, when the three-power meeting started, FDR immediately told Stalin that the Allies would invade Western France in mid-1944, without giving Churchill a chance to open his mouth. Stalin declared that this would be satisfactory. Churchill nearly burst a blood vessel, but was compelled to mutter his agreement to this fait accompli.