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My API level in Unity is 0

I have installed android-sdk on my computer. My API level is 27 but when I want to build an android app in Unity I have an error that says The minimum API level you need is 23 but the android API level on my computer is 0.
I have uninstalled and reinstalled the android SDK. I changed the name of my android folder in the tools folder from android 8.0.1 to android_23 and android_27.
I changed the folder of android SDK.
I don't have android studio on my PC.
I don't know what to do. Help Please.

Remove SDK platform above then 23. Minimum(Maximum) install API should be 23. don't install SDK Platform greater then 23, You have APi 27, so uninstall it and install 23.

Android Studio doesn't see all API platforms

Today I decided to move from the manual Android SDK Tools and IntelliJ IDEA to AndroidStudio. So I uninstalled everything (on a Mac OS X Yosemite) and install AndroidStudio.
By default, AndroidStudio installs API platform 21. I want my project to work with API platform 19, so I opened the SDK Manager from the Tools menu and installed the API 19 SDK and images.
However, when I open the Project Structure window to select an android SDK, the Module SDK combo box only shows API platform 21. I've closed and reopened Android Studio, and still - just one API platform is listed.
What can be the problem?
UPDATE:
Manually selecting the API 19 folder and adding it as a new SDK did not work. However, deleting the API 21 SDK did work - after that the API 19 SDK appeared. I reinstalled API 21, but sure enough - only API 19 appears.

Unable to use ACompat v7 below version 20

I am not sure when this problem actually started.
But today when i tried to create a project with following settings it gives error
My IDE is Eclipse with ADT Plugin
min sdk 8 , target SDK 19 , Compile-with 19
sdk tools 23.0.5
sdk platform tools 21
sdk build tools 21.1
the project was created , but it shows a lot of errors
when i change the the target and compile-with to 20 all errors are gone.
( I also need to delete the support-v4 library that is added to projects automatically as it is a different version than the one in the appcompat-v7 )
Any help is fixing this mess would be really helpful.
So far i tried updating the sdk tools using using sdk manager , it only made things worse , so i went back to the older version .
So Now i have everything updated in my sdk manager, even the system-images. I noticed that when i try to create a new project , even though the sdk manager shows i have 5.0.1, i can only create projects upto API-21 4.x L Preview, is this normal or related to my issue.

Tested on my environment
min sdk 8, target sdk 19, compile with 19, sdk tool 24.0.2, platform tool 21, build tool 21.1.2
After delete the duplicated android-support-v4.jar, every works fine. The project can get compiled and run without any problem.
Did you update your appcompat-v7 to the latest version? In my settings
Android support repository 10, Android support library 21.0.3

"parseSdkContent failed Could not initialize class android.graphics.Typeface" in SDK Andorid 5.0

I'm getting the below error after I update the SDK Android 5.0 (API 21)
"parseSdkContent failed Could not initialize class android.graphics.Typeface"
Another thread suggested to delete .android folder, yes its works, but after sometime this error will trigger again and not able to select any Project Build Target SDK, (its allows to check the target and apply, but again open Properties->android, its showing unchecked) to build the app.
I'm using Android SDK Build Tools Rev 19.1 and Eclipse Juno. ADT version 23.0.4
Note : If my Android Project target SDK version is 20 or 19, there is no problem.

This is android package error,its corrupted android 5.0 sdk platform. Just Delete the folder <sdk>/<platforms>/<android-21> and download the android 5.0 sdk platform from sdk manager.

Got the fix. This happened because you installed a new Eclipse and loaded the 5.0 API Directly. You just have to delete the file:
".android in C:\User\\" in Windows.
After this step just install the ANDROID SDK BUILD-TOOLS revision 21
Hope it works for you :)

Is 'transition' an invalid resource directory name on Android?

I make the ApiDemos project from Android Sample Project. Unfortunately, the project complains invalid resource directory name transition /ApiDemos/res line 1 Android AAPT Problem.
My Eclipse is Version: Juno Service Release 2 and JDK 7. The ADT plugin is V22.6.0. What's wrong with it?

Yes, transition resources are valid as of API 19.
To get rid of the error, make your build SDK API level 19 or higher.
To do that, in Eclipse/ADT, either
open Project Properties, go to Android and select Project Build Target as API 19, or
edit project.properties at project root and edit target to e.g. android-19
Of course, you will have to have SDK 19 installed with SDK Manager and have up-to-date build tools installed.

In case another bonehead like me comes here: I didn't have version 19 of the API installed :P
Open up your SDK manager and download any necessary version. I just had the latest version installed (22?).
Also, this was for Xamarin for me. But it could be the case for others, too.