Although this is just an notice and doesn't affect the payment process, but raises the "access denied error" which may mislead the end user.
Notice: Undefined index: checkout_page in commerce_paypal_ec_review_pane_checkout_form_submit() (line 188 of /home/nithu/public_html/

This happens when billing information is entered at Paypal instead of on-site checkout form(which is working fine).
option "Update billing customer profiles with address information the customer enters at PayPal." is enable in EC configuration.After payment , when paypal returns to checkout page only Name and Country is showing in billing information pane.