I logged on to my PayPal account and then created my button. I selected and grabbed all codes and pasted it to my WordPress website. Everything was looking good until I clicked on the button. Boom.. Error Detected – We are unable to decrypt the certificate id
I searched a lot and made lots of changes in my code and eventually found the problem. The encrypted messy codes is ending with "—–>". When you paste the code to your wordpress editor, it makes a big empty gap between the code and "—–>". To fix the issue just remove the gap and join "—->" to the encypted messy code. Yess.. you are done.
BTW, If you need to open up your Paypal page in a new window, just do target="_blank". It works for me!