Q: My abs motor relay is missing. Would that prevent my car from starting

If you are sure it is the ABS relay missing, then that will not prevent your car from starting. That circuit is unique only to the brake system.

If the engine does not crank over fast when starting, you likely have a weak battery or another connection issue.

If the engine cranks at normal speed, then something else is the culprit. If the vehicle was jump started, it may have damaged a computer or sensor. A [code scan of the computer would be a starting point after a visual inspection. That may give you insight as to what is keeping the engine from running. Without more history or details, determining the fault is difficult and will require an in-person diagnostic of the starting issue