Error message re exceeding of user numbers

I have just tried to register an addtional PC against my product key but have been told I have exceeded the number of users allowable under the license. The product I purchased should cover up to 3 PCs per household - I have only registered ONE other computer.

 

How can I resolve this issue? I paid £64.99 for this product on 8.9.10.

 

I await your response.