I did that. And it still is in demo mode. By the way, what is the difference between the demo mode and the activated one?

I have the same error:

"Activation failed. Invalid product key. Make sure that the product key belongs to the intended product or product version"

I asked SDL Support to reset my license (because I changed computers) and now reinstalled v.5.2.0 on a new machine and I can't activate. 

Under the product key in my account, it says "BaccS Desktop 5"

I resolved this with offline activation:

In your SDL account, next to the license, click on "Can't activate online? – Get an offline Activation Certificate" and follow the steps. It worked for me

In my SDL account, next to the license, it says "Can't deactivate online? – Deactivate offline". What do I do? Should I return the license and then try to activate? Any comments or suggestions are welcome.

I still have the problem. Can you please help?

Hello, Gan,

Sorry, can't help you here. To solve licensing issues, please ask support team by logging a ticket at https://www.sdl.com/support/