[2018.3.0f2] [Error] PlayerSettings Validation: Requested build target group (18) doesn't exist; #define symbols for scripting won't be added

When clicking on “Apply in Optimization”
Should be fixed in

[Error] PlayerSettings Validation: Requested build target group (18) doesn't exist; #define symbols for scripting won't be added.
UnityEditor.PlayerSettings:SetScriptingDefineSymbolsForGroup(BuildTargetGroup, String)
OptimizationHandler.SetDefineSymbols()    Assets/AstarPathfindingProject/PackageTools/Editor/OptimizationHandler.cs:77
75:       foreach (var pair in symbols) {
76:           var defineString = string.Join(";", pair.Value.Distinct().ToArray());
-->77:           PlayerSettings.SetScriptingDefineSymbolsForGroup(pair.Key, defineString);
78:       }
79:   }

OptimizationHandler.EnableDefine()    Assets/AstarPathfindingProject/PackageTools/Editor/OptimizationHandler.cs:87
85:           return pair.Value;
86:       });
-->87:       SetDefineSymbols(newSymbols);
88:   }

OptimizationHandler.ApplyDefines()    Assets/AstarPathfindingProject/PackageTools/Editor/OptimizationHandler.cs:143
141:   foreach (var define in defines) {
142:       if (define.enabled) {
-->143:           EnableDefine(define.name);
144:       } else {
145:           DisableDefine(define.name);

AstarPathEditor.DrawOptimizationSettings()    Assets/AstarPathfindingProject/Editor/AstarPathEditor.cs:423
421:   if (GUILayout.Button("Apply", GUILayout.Width(150))) {
422:       if (EditorUtility.DisplayDialog("Apply Optimizations", "Applying optimizations requires (in case anything changed) a recompilation of the scripts. The inspector also has to be reloaded. Do you want to continue?", "Ok", "Cancel")) {
-->423:           OptimizationHandler.ApplyDefines(defines);
424:           AssetDatabase.Refresh();
425:           defines = null;

AstarPathEditor.DrawMainArea()    Assets/AstarPathfindingProject/Editor/AstarPathEditor.cs:382
380:   DrawSettings();
381:   DrawSerializationSettings();
-->382:   DrawOptimizationSettings();
383:   DrawAboutArea();

AstarPathEditor.OnInspectorGUI()    Assets/AstarPathfindingProject/Editor/AstarPathEditor.cs:260
258:   string storedEventCommand = Event.current.commandName;
-->260:   DrawMainArea();
262:   GUILayout.Space(5);



Thank you for reporting this. Unity unfortunately does not expose which build targets are deprecated and which ones are not, so every time Unity deprecates a build target I have to manually add an exception to my code.
Everything will work without any issues in your version except that it will log that error message.
This will be fixed in the next version.