Serial number problem

I purchaed Adobe LightRoom Elements 2019 at Bestbuy in DVD form and loaded it onto my computer as well as  set up an account at Adobe.com.  My problem is that when I set up my Adobe.com account, it asked for my serial number (which was provided on the disk cover) but would not let me enter it (the next button was greyed out).  I'm afraid that means that when I need to replace the computer I'm using (which will probably happen sooner rather than later as it's pretty old) and try to download it again, onto the new computer, that the sysem will not let me becuase the serial number hasn't been registered.  Any suggestions?  I can't even find in my Adobe.com where the serial number is....