Why did britain and france choose not to attack germany in 1939 and early 1940 what were the possible disadvamtages?

They were not prepared for war.  The greatest possible disadvantage was that if they had tried to attack Germany at that time, they may well have lost.  They needed time to ramp up for full military action.

After "the war to end all wars" (World War I, which was then called The Great War), there had been a strong aversion to return to war.  The world was not ready for the way Germany remilitarized itself and used Blitzkrieg tactics to embark on a new war.  When Germany attacked France in May, 1940, France was overwhelmed.  The British needed to hold on with much resolve during the Battle of Britain (July - October, 1940).  So to think that the British or French were ready in 1939 or 1940 to take on the German war machine on German soil would have been potentially disastrous.

It might be the second one