Trouble with a critical main engine computer forced NASA managers to cancel the planned launch of the shuttle Atlantis yesterday.A spare unit is available at the Kennedy Space Center and its installation likely will delay launch a week or so based on similar problems in the past. The goal of the 42nd shuttle mission, the fourth of six planned for 1991, is the deployment of a $120 million Tracking and Data Relay Satellite - TDRS - nestled in Atlantis's 60-foot-long cargo bay. The 5,000-pound satellite is the fifth in a series of advanced solar-powered relay stations used to route communications between ground stations and a multitude of civilian and military spacecraft, including NASA's Hubble Space Telescope and the recently launched Gamma Ray Observatory astronomy satellite. The flight plan called for launching of the satellite six hours and 13 minutes after the shuttle's takeoff. The astronauts plan to devote the rest of their mission to carrying out a smorgasbord of minor on-board science and engineering experiments, including six devoted to learning more about how to combat the adverse medical effects of weightlessness. Landing is scheduled for the Kennedy Space Center, the first planned Florida landing since 1985, when the shuttle Discovery suffered major tire and brake damage during touchdown in a stiff crosswind.