How to get sales order as option in customize form

how to get sales order as option in customize form

I am following the video by erpnext foundation to create custom application, where i had created new app as erpnext_shipping. But when im trying to customize sales order form , the option for sales order is not there in drop down it asks me to create one. As per video it should be there

Solved, i forgot to install erpnext first and install app
as im very new to frappe please help to explore