https://discuss.frappe.io/t/multi-tenant-setup-on-bench/22366/6
This line seems to resolve this issue.
frappe@erpnext:~/frappe-bench$ sudo supervisorctl reload
Restarted supervisord
frappe@erpnext:~/frappe-bench$ sudo service nginx start
which requires supervisor to be installed, which for some reason, I missed.
sudo apt install supervisor