如果上面两个答案没用的话，试试我这个。
If those two answers are useless,Try mine.
这个问题我花了大半天时间来搜索解决方案。终于找到了有用的东西
I spent almost half day to search for the solution to this.Finally, I found something useful.
可以参考一下上面两个问题的答案，其实两个都一样。
You can refer these two links above. Both the same, either one will be fine.
为了方便你们，我直接复制了解决方案：
For your convenient, I copyed it :
This is a known issue with changing to 4.12 and using tasks. It can be resolved by simply adding “GameplayTasks” to the PublicDependencyModuleNames section inside your ‘ProjectName’.Build.cs For example:
PublicDependencyModuleNames.AddRange(new string[] { “Core”, “CoreUObject”, “Engine”, “InputCore”, “AIModule”, “GameplayTasks” });
在YourProjectName.Build.cs里的PublicDependencyModuleNames加上GameplayTasks就行。
Add GameplayTasks to PublicDependencyModuleNames of the YourProjectName.Build.cs file~.