When affected by the Slow spell:
An affected target’s speed is halved, it takes a -2 penalty to AC and Dexterity saving throws, and it can’t use reactions. On its turn, it can use either an action or a bonus action, not both. Regardless of the creature’s abilities or magic items, it can’t make more than one melee or ranged attack during its turn.
A fighter is affected by Slow. Can they use Action Surge for two actions (attacks) in a turn?
Action Surge - Starting at 2nd level, you can push yourself beyond your normal limits for a moment. On your turn, you can take one additional action.

