Invalid Serial Number for Lightroom 5.7.1

Hello,

My 7 years old iMac HDD has just died on me and I've got the HDD replaced and trying to reinstall all of the necessity apps back to work. 

I have the serial number for LR5 I bought in 2015, but for some unknown reason I couldn't use it to activate the LR version 5.7.1.  As I attempt to activate the software, LR5.7.1 threw me an error message stating that my serial number is invalid. The installer for LR 5.7.1 was downloaded from the URL provided by Adobe support. I have tried to ask help from the support representative but he told me that they are no longer giving any support for older version of LR, and advised me to seek help at this forum.

Has anyone here had experienced this kind of problem before and how do you address it? 

I'm thinking that the serial number of LR5.0 might not be working on the slightly newer version 5.7.1, does any one here has the download link for version pre-5.7.1?

Anyway, thanks heap in advance for responding.

Caesar