I reinstalled my laptop from scratch, then installed Lightroom 5 demo. The setup went through adobe creative cloud tool (was not that way before), I obtained LR 5.5
Then I try to activate it using the serial number I'va had for a while now and it is rejected as not being valid. This is the same SN I used befoore and this is the same I see when I check my account information on Adobe website. I made sure this was not a typo.
Uninstall the Lightroom version you downloaded for the Creative Cloud which only gets licensed when you subscribe to the payment plan.
Product updates this link will take you to the updates site for all Adobe Products. Download the appropriate Lightroom 5 version i.e 5.5 then your license key should work. If your LR 5 was an upgrade purchase you will also need the key from the earlier version to also input if prompted.
That worked like a charm, thank you very much for your great answer.
You are welcome.