Why did my unity project cannot be saved as apk?

Hi, I am new to Unity. I tried to create a simple game ( honestly, its just some pieces of scripts I read on a tutorial book ). When I tried to save my game as .apk with File > Build Settings > Build i found this error:

CommandInvokationFailure: Unable to list target platforms. Please make sure the android sdk path is correct. See the Console for more details. C:/Program Files/Java/jdk1.8.0_66\bin\java.exe -Xmx1024M -Dcom.android.sdkmanager.toolsdir="D:/AndroidSDK/Android-sdk\tools" -Dfile.encoding=UTF8 -jar "C:\Program Files\Unity\Editor\Data\PlaybackEngines\AndroidPlayer/Tools\sdktools.jar" -

stderr[ Error:Invalid command android ] stdout[

] UnityEditor.Android.Command.Run (System.Diagnostics.ProcessStartInfo psi, UnityEditor.Android.WaitingForProcessToExit waitingForProcessToExit, System.String errorMsg) UnityEditor.Android.AndroidSDKTools.RunCommandInternal (System.String javaExe, System.String sdkToolsDir, System.String[] sdkToolCommand, Int32 memoryMB, System.String workingdir, UnityEditor.Android.WaitingForProcessToExit waitingForProcessToExit, System.String errorMsg) UnityEditor.Android.AndroidSDKTools.RunCommandSafe (System.String javaExe, System.String sdkToolsDir, System.String[] sdkToolCommand, Int32 memoryMB, System.String workingdir, UnityEditor.Android.WaitingForProcessToExit waitingForProcessToExit, System.String errorMsg) UnityEditor.HostView:OnGUI()

I have checked my SDK Manager ( SDKManager.exe ) and found that there are a lot of components "not installed", is this the problem? I use Android Studio to download and install the SDK, why did there are still a lot of components "not installed" and when I tried to install it with SDKmanager.exe I have to download it again?

Please tell me what should I do,

Thanks in advance, herautama