When you open the theme in the substrate app, you’ll see the drop-down menu at the top of the overlays. You must choose one based on your Android ROM version. There are also overlays with different variants for a specific Android version. You need substratum or substartum lite (for Android Q) app from the Play Store to apply the theme
Supported Android
{7.0 and UP}
Supported Android Version:-
Nougat (7.0 – 7.1.1) – Oreo (8.0-8.1) – Pie (9.0)