When I try to export the game build for the Android platform, I started getting this error:
Within the Project Settings - Minimum API Level and Target API Level not get loaded anyhow! While I have used all default Unity provided settings to export Android build. Here is the image to illustrate this:
I have already read all the threads related to the same problem but overall I can’t able to find the solution that actually worked for me.
Google suggests that you have JDK 9 installed on your system, either uninstall it or try the workarounds described here - android - Failed to run sdkmanager --list with Java 9 - Stack Overflow
Thank you for your quick reply 
I have tried to follow the given answer at my best but still, I can’t able to get success.
I want to share screenshots after applying the solution what happened with me:
sdkmanager script updated screenshot:
In the last point, the person said to run the command but this is happening to me:
I am noob in executing commands in Terminal so I don’t know what is going wrong!!
So please suggest me further into this.
I have just downloaded all things from the Unity HUB then also this kind of problem occurring that I can’t able to understand.
NOTE: Recently I have upgraded my Java SE, maybe then after problem get started:
Did you try simply uninstalling the system JDK? And reverting back to JDK 8
Do you suggest me to remove this version?
I have started downloading JDK 8 version:
I hope after I remove JDK 13, it will not start creating a problem for other Unity versions.
Because other 2 Unity versions working properly at present
Which Unity versions are you referring to? if you want to be on the safe side, just have JDK 8 installed.
According to Java 9 (JDK 9) support by Unity Android , no external JDK are required. So it should be safe to remove.
I have installed JDK 8 into my computer and restart it too.
But still, the problem is not solved 
Library and SDK linked to all default places.
when you open terminal and do
java -version
what does it say?
Hmph, well a long shot, but you could try downloading Android Studio, and see if you can build to Android from there.
If yes, in Android Studio change SDK path to point to the on Unity installation path, and try building to Android again
Thank you to remain with me in my this problem
I have already in my system Android Studio exist with this kind of SDK and other packages download:
Now Unity can’t able to accept this Android Studio SDK and the overall situation remain the same.
I mean the other way around, change Android SDK path in Android Studio to point to the one Unity has in its installation path
Okay, I have performed the reverse step and try to link Unity SDK with Android Studio.
This is the first step after linking:
There is some installation required so I have completed that:
So this is the situation after Android Studio linking:
What is the next step I require to do?
Select API Level 28, and install it, currently it says it’s Partially installed, which is incorrect
Yes sir, now API get linked with the SDK successfully and on the spot, library gets loaded in the Unity Project Settings:
Exactly what is the problem?
Partial SDK or wrong JDK version…
I would guess something with your OS wrong, maybe as soon as you installed newest JDK, it did something… but can’t be sure what.
Glad it works for you now
Could you please do this
Create NewBehaviourScript.cs, attach it to some GameObject and run
using System;
using System.Collections;
using System.Collections.Generic;
using UnityEngine;
public class NewBehaviourScript : MonoBehaviour
{
// Start is called before the first frame update
void Start()
{
}
// Update is called once per frame
void Update()
{
}
private void OnGUI()
{
GUILayout.Label(Environment.GetEnvironmentVariable("JAVA_HOME"));
}
}
I think your JAVA_HOME env variable is incorrect, it should point to OpenJDK inside Unity.