title: application cannot be installed on API 21
body: <p>I just made an application using Android Studio 3.6, I tested using API 29, the application can be installed, but when I ask my friend to test on his device (Android 6.0) the application is not installed, so my application can only be installed on Android 7.0 and above, I want old Android users to be able to use my application. This is build.gradle that I use</p>
  
  <pre><code>apply plugin: 'com.android.application'
  apply plugin: 'com.google.gms.google-services'
  
  android {
      compileSdkVersion 29
      buildToolsVersion "29.0.3"
  
      defaultConfig {
          applicationId "com.my.app"
          minSdkVersion 21
          targetSdkVersion 29
          versionCode 10
          versionName "1.0"
  
          testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
      }
  
      buildTypes {
          release {
              minifyEnabled false
              proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
          }
      }
  
  }
  
  dependencies {
      implementation fileTree(dir: 'libs', include: ['*.jar'])
  
      implementation 'androidx.appcompat:appcompat:1.1.0'
      ...
  }
  </code></pre>
  
  <p>I have tried to downgrade the target SDK but it still cannot be installed</p>