I cannot use my purchased version of Movie Studio 16.0 Platinum. I used to be able to use it on this same device, but now when I start the software I get the following error.

Serial number is already used.

I see that the product is activated and assigned to this device. I removed it from the list on my account online; however, I get the same error. How can I edit existing projects that I created if you don't let me use the software I paid for? I'm using the correct sign-in info and serial number.