In 2015 I bought Sony Vegas Pro, I am retiring the w7 machine that it resides on and want to install on my new w10 machine.

I have the purchase receipt e-mail from OrderFulfillment@dstewart.com where I bought the software

When I put the code in to access my download the message says "The entered voucher code is invalid or has already been used. Please check your entered information.

Do I need to unauthorize it first on the older computer, if so how do I do this?

Thanks