I also faced this issue you just need to make 2 changes:
File Name : android/build.gradle mention this below code
subprojects {
afterEvaluate {
project -> if (project.hasProperty("android")) {
android {
compileSdkVersion 26 buildToolsVersion '26.0.2'
}
}
}
}
File Name :android/app/build.gradle change your compliesdk version and buildToolVersion like this:
compileSdkVersion 26 buildToolsVersion "26.0.2"
and in
dependencies {
compile 'com.android.support:appcompat-v7:26.0.2'
}