BEST ANSWER IS TO USE UBUNTU VERSION COMPATIABLE WITH VESTACP NOT LATEST
Pulling private repository requires SSH authentication for which you have to create a SSH key in your server which can be done using this code :
ssh-keygen -t rsa -b 4096 -C "server@restro.io" ( server@restro.io is unique server identifier )
cat ~/.ssh/id_rsa.pub ( this will print SSH key which you need to add into github )
Repository > settings > Deploy Keys > add here