Got it fixed.
In IPN settings tab select all Response Tags. By default only few are selected.
On webhooks & API tab.
Uncheck "Enable return after sale"
In Notifications section: select "Email Text and IPN" from the dropdown menu. Default is "Email Text" only.
What is the url of Thank you page? On 2checkout website in Webhooks and API page "
Redirect your shoppers to a custom URL after payment confirmation. Only applicable to online payment methods (ex: Credit cards and PayPal)."
section is blank.