So I bought Photostory Deluxe 2019 about a year ago and do to some computer issues I had to reformat my hard drive. When I try to register Photostory after reinstallation it says the serial number has already been used.
Using your web browser go to the Magix main web site (magix.com) and click the My Account icon and login in with your email address and password you originally registered the software with.
Select My Products, there you will see download links and your serial number - click the Show more details link - this will take you to the Activation slots information.
If you have Photostory installed on 2 computers, select the one you have re-installed on and de-activate it, then try registering/activating again.
. . . . My product is not listed and when I try to add it it says serial number has already been used. . . . .
Assuming you had installed and activated/registered the software previously, this would suggest that the software was registered previously with a different email address, or there was an error in the email address it was registered with.