SystemUI Tuner has some unconventional configurations that require special steps to be able to build.
First, SystemUI Tuner makes use of hidden APIs in Android. To avoid reflection, a special SDK JAR is used to directly access these APIs. To successfully build, you'll need to grab the Android 10 (API 29) JAR from here, and follow the instructions to install it.
Next, there are some locally-referenced dependencies. These dependencies are also available remotely, but they need to be replaced. Check the app-level build.gradle and the settings.gradle files for instructions.