Short answers, pulled from the story.
The Army of England (Armée d'Angleterre) was a field army of the French Revolutionary Army established on the 26th of October 1797 by the French Directory. Its original purpose was the invasion of Great Britain, though that invasion never took place.
Napoleon Bonaparte was appointed commander-in-chief on the 26th of October 1797, with Louis Desaix serving as provisional commander in Paris during Bonaparte's absence. The army subsequently passed through at least nine distinct command arrangements, ending with Gabriel de Hedouville as the final commander.
By the end of 1797-40,000 men from the Army of Italy and 10,000 each from the Army of the Rhine and the Army of Mainz had been called to the new force, totaling around 60,000 men, though the army had not yet physically assembled.
The source does not give a specific reason for the abandonment of the invasion plan. The plan was eventually dropped and the army was redirected to suppress the Chouannerie of 1799, an uprising in Western France.
On the 14th of January 1800, the Army of England was renamed the Army of the West. The order took effect on the 17th of January 1800, ending the army's existence under its original name.
The Chouannerie of 1799 was an uprising in Western France. After the British invasion plan was abandoned, the Army of England was reassigned to suppress this domestic rebellion rather than carry out any cross-Channel operation.