I think you’ve already installed frappe-bench.
You probably ran something like this
bench init frappe-bench --frappe-branch master && cd frappe-bench
To add erpnext, the commands are
bench new-site site1.local
bench start
then you’d add (as user frappe)
cd frappe-bench
bench get-app erpnext GitHub - frappe/erpnext: Free and Open Source Enterprise Resource Planning (ERP) --branch master
bench --site site1.local install-app erpnext
bench start