Android Studio 3.1.3 having issues with Constraint layout

Had the same issue - went to SDK manager and installed additional SDK platforms; Oreo 8.1. It seems the issue was that the 'new' API 28 still has issues. It says partially installed, but I basically just checked the boxes for lower API's and it downloaded/installed:

Additionally, I changed my build.gradle file's SDK version, buildTools, appcompat, and design versions as below. It works now and I will move back to API 28 at a later stage when these issues have been resolved.

apply plugin: 'com.android.application'

android {
compileSdkVersion 27
buildToolsVersion '27.0.3'
defaultConfig {
    ...
    minSdkVersion 15
    targetSdkVersion 27
    ...

dependencies {
implementation 'com.android.support:appcompat-v7:27.0.0'
implementation 'com.android.support.constraint:constraint-layout:1.1.2'
implementation 'com.android.support:design:27.0.0'
}

You just need to change the following codes from the dependencies section of your build.gradle file:

From:

implementation 'com.android.support:appcompat-v7:28.0.0-rc02'
implementation 'com.android.support:design:28.0.0-rc02'

To:

implementation 'com.android.support:appcompat-v7:28.0.0-alpha1'
implementation 'com.android.support:design:28.0.0-alpha1'

And then sync your project.