I have few problems with android studio

1, I am trying to put an image on my design but it doesn't show up. I put the code in and the code is...
<ImageView
android:layout_width="match_parent"
android:layout_height="300dp"
android:background="@drawable/blue_background"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
tools:layout_editor_absoluteY="128dp"
tools:ignore="MissingConstraints"
android:contentDescription="@String/todo"/>

2. Why Emulator isn't showing my app what I create in android studio?
I created my app but for some reason it isn't showing up on emulator screen instead it shows normal phone screen.

3. Compilation errors "This project contains Java compilation errors, which can cause rendering failures for custom views. Fix compilation problems first."
Please help me

 

It will be the same and have a error message.

 

I fixed the error message but the problem is, it doesn't have my app in emulator

 

Where can I write this code? I'm in build.gradle (module: Second app.app)

plugins {
id 'com.android.application'
}

android {
compileSdkVersion 30
buildToolsVersion "30.0.3"

defaultConfig {
applicationId "com.example.secondapp"
minSdkVersion 23
targetSdkVersion 30
versionCode 1
versionName "1.0"

testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
}

buildTypes {
release {
minifyEnabled false
proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
}
}
compileOptions {
sourceCompatibility JavaVersion.VERSION_1_8
targetCompatibility JavaVersion.VERSION_1_6
}
}

dependencies {
implementation 'androidx.appcompat:appcompat:1.2.0'
implementation 'com.google.android.material:material:1.2.1'
implementation 'androidx.constraintlayout:constraintlayout:2.0.4'
testImplementation 'junit:junit:4.13.1'
androidTestImplementation 'androidx.test.ext:junit:1.1.2'
androidTestImplementation 'androidx.test.espresso:espresso-core:3.3.0'
}