Android – Unable to resolve dependency for ‘:app@debug/compileClasspath’: Could not resolve com.android.support:appcompat-v7:26.1.0

Finally I fixed the problem by modifying build.gradle like this:

android {
    compileSdkVersion 26
    buildToolsVersion "26.0.2"

    defaultConfig {
        minSdkVersion 16
        targetSdkVersion 26
    }
}

dependencies {
    implementation fileTree(dir: 'libs', include: ['*.jar'])
    implementation 'com.android.support:appcompat-v7:26.1.0'
    implementation 'com.android.support.constraint:constraint-layout:1.0.2'
    implementation 'com.android.support:design:26.1.0'
}

I've removed these lines as these will produce more errors:

testImplementation 'junit:junit:4.12'
androidTestImplementation 'com.android.support.test:runner:1.0.1'
androidTestImplementation 'com.android.support.test.espresso:espresso-core:3.0.1'

Also I had same problem with migrating an existing project from 2.3 to 3.0.1 and with modifying the project gradle files like this, I came up with a working solution:

build.gradle (module app)

android {
    compileSdkVersion 27
    buildToolsVersion "27.0.1"

    defaultConfig {
        applicationId "com.mobaleghan.tablighcalendar"
        minSdkVersion 16
        targetSdkVersion 27
    }

dependencies {
    implementation 'com.android.support:appcompat-v7:25.1.0'
    implementation 'com.android.support:design:25.1.0'
    implementation 'com.android.support:preference-v7:25.1.0'
    implementation 'com.android.support:recyclerview-v7:25.1.0'
    implementation 'com.android.support:support-annotations:25.1.0'
    implementation 'com.android.support:support-v4:25.1.0'
    implementation 'com.android.support:cardview-v7:25.1.0'
    implementation 'com.google.android.apps.dashclock:dashclock-api:2.0.0'
}

Top level build.gradle

buildscript {
    repositories {
        google()
        jcenter()
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:3.0.1'
    }
}

allprojects {
    repositories {
        google()
        jcenter()
    }
}

Check your internet connectivity. For me it was just internet connectivity problem. If you are under a proxy server. Go to File > Settings > Appearance & Behavior >System Settings >HTTP Proxy and enter your proxy details there. Use the check connection option to verify your connection. Here is a reference to my gradle file.

apply plugin: 'com.android.application'

android {
    compileSdkVersion 28
    defaultConfig {
       applicationId "com.oops.pk.birthday_card"
       minSdkVersion 23
       targetSdkVersion 28
       versionCode 1
       versionName "1.0"
       testInstrumentationRunner 
       "android.support.test.runner.AndroidJUnitRunner"  
    }
    buildTypes {
         release {
             minifyEnabled false
             proguardFiles getDefaultProguardFile('proguard-android.txt'), 
             'proguard-rules.pro'
          }
     }
}

dependencies {
     implementation fileTree(dir: 'libs', include: ['*.jar'])
     implementation 'com.android.support:appcompat-v7:28.0.0-rc02'
     implementation 'com.android.support:support-v4:28.0.0'
     implementation 'com.android.support:recyclerview-v7:28.0.0'
     implementation 'com.android.support:design:28.0.0'
     testImplementation 'junit:junit:4.12'
     androidTestImplementation 'com.android.support.test:runner:1.0.2'
     androidTestImplementation 'com.android.support.test.espresso:espresso- 
     core:3.0.2'
 }