Since python2.7 support is not more in picture, we are facing problem while installing virtualenv at folder for develop. by default version python is 2.7 in system then you will face issue while installing version-11 version-12 Frappe Erpnext , this is one of the possibility.
Hence use below commands for
Frappe , Erpnext version > 11: virtualenv -p /usr/bin/python3 . Frappe, Erpnext Version <11: virtualenv -p /user/bin/python2 .