You can update your cordova-android to 7.0.0 as that's the version that uses SDK 26 as target, or you can update to cordova-android 7.1.4, as it targets SDK 27.
You can do it by removing the platform and adding it again specifying the version to use like this: (if not using ionic, remove the ionic word from both commands)
ionic cordova platform rm android
ionic cordova platform add android@7.1.4
If you don't want or can't update, you can try to set the target SDK with this preference in the config.xml
<preference name="android-targetSdkVersion" value="26"/>
And then run ionic cordova prepare
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