I have not found the SlicerANTs extension in the extension manager (I use Slicer 4.11 version), Furthermore, when I install it via GitHub I cannot find it anywhere among extensions.
SlicerAnts is not available for the latest stable release (Slicer 4.11.20210226). You will have to use the latest Slicer Preview release which you can download from https://download.slicer.org/. Using this latest version you will be able to install SlicerAnts from the Extensions Manager.