Failed to resolve: com.android.support:appcompat-v7:28.0

28.0.0 is the final version of support libraries. Android has migrated to AndroidX. To use the latest android libraries, Migrating to AndroidX

Edit: Versions 28.0.0-rc02 and 28.0.0 are now available.

I don't see any 28.0 version on Google Maven. Only 28.0.0-alpha1 and 28.0.0-alpha3. Just change it to either of those or how it was previously, i.e., with .+ which just means any version under 28 major release.

For an alpha appcompat release 28.+ makes more sense.

Add the following code on build.gragle (project) for adding Google maven repository

allprojects {
    repositories {
    ...
        maven {
            url 'https://maven.google.com/'
            name 'Google'
        }
    ...
    }
}

some guys who still might have the problem like me (FOR IRANIAN and all the coutries who have sanctions) , this is error can be fixed with proxy i used this free proxy for android studio 3.2 https://github.com/freedomofdevelopers/fod just to to Settings (Ctrl + Alt + S) and search HTTP proxy then check Manual proxy configuration then add fodev.org for host name and 8118 for Port number

As @Sourabh already pointed out, you can check in the Google Maven link what are the packages that Google has listed out.

If you, like me, are prompted with a similar message to this Failed to resolve: com.android.support:appcompat-v7:28.0, it could be that you got there after upgrading the targetSdkVersion or compileSdkVersion.

What is basically happening is that the package is not being found, as the message correctly says. If you upgraded the SDK, check the Google Maven, to check what are the available versions of the package for the new SDK version that you want to upgrade to.

I had these dependencies (on version 27):

implementation 'com.android.support:appcompat-v7:27.1.1'
implementation 'com.android.support:design:27.1.1'
implementation 'com.android.support:recyclerview-v7:27.1.1'
implementation 'com.android.support:cardview-v7:27.1.1'
implementation 'com.android.support:support-v4:27.1.1'

And I had to change the SDK version and the rest of the package number:

implementation 'com.android.support:appcompat-v7:28.0.0'
implementation 'com.android.support:design:28.0.0'
implementation 'com.android.support:recyclerview-v7:28.0.0'
implementation 'com.android.support:cardview-v7:28.0.0'
implementation 'com.android.support:support-v4:28.0.0'

Now the packages are found and downloaded. Since the only available package for the 28 version of the SDK is 28.0.0 at the moment of writing this.