I could be wrong but I think that the "Professional Plus" version of Office is for installations within corporate environments and whilst it is possible to buy that version online it can lead to the problems that you are having when it comes to installing it. It is a well known issue when buying Office 'cheap' online.

As I understand it organisations buy a number of "Professional Plus" licenses all using the same key and when that number has been installed then no further installs are allowed. The organisation may not use all the licenses and some find their way for sale on the internet and some unscrupulous vendors will sell more licenses than are remaining. Maybe you bought one of these.

I sincerely hope that this is not the case and that you resolve this to your satisfaction. Good luck.