I fixed failure transaction issue. However checkout.php looks like not communicating with Paypal Gateway at all. Paypal Express button doesn't do anything either. Username, password, signature are from Paypal profile/API credential. I copied them and pasted on the fields in checkout insert form interface. My shopping site is : waterfiltershopping.php
I'm not using sandbox and instead put a temporary $1.00 item at the bottom.
When I used the checkout wizard, cart was working properly, however I didn't like redundancy of the checkout given and needed to simplify the process using local checkout. Best regards,