Wrap with UNITY_EDITOR.

UnitySDK/Assets/ML-Agents/Scripts/RewardProvider/LowLevelRewardProviderComponent.cs

 using System;
+#if UNITY_EDITOR
+#endif
 using UnityEngine;
 
 namespace MLAgents.RewardProvider
 
         void RewardReset(float reward)
         {
+#if UNITY_EDITOR
             var keyframe = new Keyframe
             {
                 time = Time.realtimeSinceStartup,
             var index = rewardCurve.AddKey(keyframe);
             AnimationUtility.SetKeyLeftTangentMode(rewardCurve, index, AnimationUtility.TangentMode.Linear);
             AnimationUtility.SetKeyRightTangentMode(rewardCurve, index, AnimationUtility.TangentMode.Linear);
+#endif
         }
     }
 }