installation and use of Lightroom

I have a Lightroom 6 on disc I purchased in Sweden Summer 2016. I tried to install the program in an older iMac with Mac OS El Capitan. But I am cought in a Moment 22. The installation went OK, but when I try to open the program it asks for "log in" with all that done, it asks for the serial number. But there is not boxes to fill in, even though the computer is online.

I used the program before on a newer iMac, but when I continued to upgrade the Mac OS my Lightroom was finally done and I had to turn to an older version of Mac OS to manage to get along with the my Lightroom.

Unhappy Adobe user.