The whole procedure is here
GitHub - openatv/enigma2: openatv-gui
openatv-gui. Contribute to openatv/enigma2 development by creating an account on GitHub.
github.com
when the line is reached
git clone https://github.com/oe-alliance/build-enviroment.git -b 5.1
you are knocking
git clone https://github.com/oe-alliance/build-enviroment.git -b 5.2
it is the latest version 7.2
Patience and persistence are important