The message that you get about the program already being registered likely has an error code included in the message. The most common error codes re this are:

Error code 14 - see https://www.vegascreativesoftware.info/us/support/article/troubleshooting-error-code-14--12/ re solving this error code.

Error code 24 - see https://www.vegascreativesoftware.info/us/support/article/how-to-solve-error-code-24--2/ re solving this error code.

If the error code is neither of these, please advise the error code number.

Please note that this a peer-to-peer user forum and not MAGIX technical support. If you want to contact MAGIX Support about your issue, you will need to follow the Support link in the header of the forum webpage though to get to lodging a support ticket is a very convoluted process.