Have you ever previously had any version of Sound Forge on your computer? If so, it would appear that SF12 is aware of this and thinks that you are trying to install the trial version of an already existing program, perhaps in order to extend the trial.
However, whether or not you have had an earlier version installed I suspect the only way to resolve this is to contact Magix Support. That page does require you to already have a Magix account, which you you may well not have if this is your first time of testing one of their products. If you do not have such an account you could try emailing support@magix.com
Err, no, but I suspect the fact that you presumably do not have a valid registration with Magix might be the problem. (Incidentally, a lot of companies now offer support on a "subscription" basis, it's not only Avid.)
You have not actually said whether you have ever previously had any version of SF installed on your computer; that might well be very relevant to your problem.
I presume that you have tried first the direct email approach and then contacting via the support web site? The only other route I can suggest at the moment is to try emailing sales@magix.com and fully explaining there your problem. At the least they should be able to possibly redirect your email to the correct destination.