A forum search turned up this:
This recent bug report confirms it is a ‘known problem’ -
In my own instances, the file is not in v8 but is in v7 however at a different path from yours -
frappe@erpnext:~/frappe-bench$ find . -name daterangepicker.css
./apps/frappe/frappe/public/css/daterangepicker.css
frappe@erpnext:~/frappe-bench$ bench version
erpnext 7.2.18
frappe 7.2.14
frappe@erpnext:~/frappe-bench$ find . -name daterangepicker.css
“nothing”
frappe@erpnext:~/frappe-bench$ bench version
erpnext 8.0.10
frappe 8.0.14
What version are you running and does it show up in a search in your instance?