I downloaded adobe Acrobat Pro DC 2015 release and purchased a software license but the system keeps saying the code is invalid when I try to enter it. How can I resolve this issue?

What is the entire message you get when it fails?

Error "The serial number is not valid for this product" | Creative Suite

http://helpx.adobe.com/creative-suite/kb/error-serial-number-valid-product.html

make sure you're using the correct number, Find your serial number quickly

then make sure you're using the matching product (ie, re-read the exact program for which the serial number is inteneded,

Error "The serial number is not valid for this product" | Creative Suite