Passed down erp instance will not update

I was given a running erpnext/frappe instance, running version 15.18.1 for erpnext and frappe 15.19.0. I planned on installing the hrms app and updating the existing applications, but when I bench update I get the error

‘subprocess.CalledProcessError: Command ‘git symbolic-ref -q --short HEAD’ returned non-zero exit status 128.’

any help with resolving this would be greatly appreciated

Plesae check it.

frappe-bench was already updated, but when I went to the erpnext folder in /apps and ran git status and got this ‘fatal: not a git repository (or any of the parent directories): .git’. Is it possible to set these as repositories now?