Dear @saurabh6790 thanks for such a brief explanation
now I got,
sync starts from Shopify not from ERPNext
what is my problem
when I save shopify-setting WEBHOOKS DETAILS section does not appear
when WEBHOOKS DETAILS section showed then it means Shopify and ERPnext got connected and should start sync.
look at the bellow screen shot there is not sign of WEBHOOKS DETAILS
do I need to do anything in shopify store except creating a private app?