The easiest way to install git-annex on Android is using Termux or Nix-On-Droid.

It's also possible to install git-annex manually, using the Linux standalone build, run inside Termux or a similar app:

pkg install wget
wget https://git-annex.branchable.com/install/Android/git-annex-install
source git-annex-install

The old git-annex Android app, is no longer bEing updated, details about it are at oldapp.