I had the same issue on my other computer, which was resolved a few months ago. I download Lightroom and when I type in my serial number, it gives me a red X, indicating that the number is incorrect. I triple-checked it, and it is not. When I spoke with the customer service rep, she guided me through a process in which I had to uninstall my current version (5.4 I believe), install an older version (4.something), and used the serial number with that version. It worked! However, I lost the transcript of our conversation and I can't remember which version I was supposed to download, leaving me unable to use Lightroom on my other computer. Anyone know what I should do?