My guess is that probably you have already an earlier, or perhaps the same, version of Photomizer already installed on your computer.
So, if it is the same version, then keep it, and do not install today’s offer.
But if your existing Photomizer is an older one, uninstall it, and then try to install today’s version.
This is how I went about it when it happened to me.
Hope it helps.
Cheers,
consuella