ERROR: Failed to resolve: com.github.LuckSiege.PictureSelector:picture_library:v2.6.0

忘记写第二个了，来指定仓库地址：

allprojects {
    
    
   repositories {
    
    
      jcenter()
      maven {
    
     url 'https://jitpack.io' }
   }
}

放置再gradle的android括号中。然后，指定minsdk为20即可。