Android - Application Development - Error (minSdk(API 23)> deviceSdk(API 19) error)

You have set minimal SDK on your project is API 23 (marshmallow) and your device or emulator has SDK(API 19, Kitkat). Just go to build.gradle(Module:app) and change minSdkVersion:

android {
    compileSdkVersion 26
    buildToolsVersion "26.0.1"
    defaultConfig {
        applicationId "YOUR PACKAGE"
        minSdkVersion 17 //change here
        targetSdkVersion 26
        versionCode 1
        versionName "1.0"
        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
        multiDexEnabled true
    }