Could not resolve com.android.support:appcompat-v7:26.1.0 in Android Studio new project

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()
    }
}

goto Android->sdk->build-tools directory make sure you have all the versions required . if not , download them . after that goto File-->Settigs-->Build,Execution,Depoyment-->Gradle

choose use default gradle wapper (recommended)

and untick Offline work

gradle build finishes successfully for once you can change the settings

If it dosent simply solve the problem

check this link to find an appropriate support library revision

https://developer.android.com/topic/libraries/support-library/revisions

Make sure that the compile sdk and target version same as the support library version. It is recommended maintain network connection atleast for the first time build (Remember to rebuild your project after doing this)

try this :

android {
    compileSdkVersion 26
    buildToolsVersion "26.0.1"
defaultConfig {

        targetSdkVersion 26
    }

}


compile 'com.android.support:appcompat-v7:25.1.0'

It has worked for me