In my case I had a frappe/bench v13 on Ubuntu 18 that stopped working after upgrading the OS to 20. I had a second vm as a backup and used that for operations. After alot of trial and error I decided I would get a working bench, then install ERPNext on a new site with v14 and run tests to try it out.
OS: Ubuntu 20.04.4 LTS
Python: 3.8.10
node: v14.19.0
yarn: v1.22.17
Mariadb: Ver 15.1 Distrib 10.3.34-MariaDB
Redis: 5.0.7
Nginx: 1.18.0
Pip3: pip 21.3.1
I suggest following the steps for installing frappe-bench for whatever OS on this page , then once you have a working bench:
bench new-site <sitename>
bench get-app --branch version-14-beta-pre-release erpnext
bench --site <sitename> install-app erpnext
As some of our wiser contributors would say: YMMV