Cannot activate Norton Online Security

My trial period for NOS recently expired and I followed the prompts to purchase the product. I have downloaded the product but now I am stuck at the activate stage. I have copied and pasted the Product Key but the "Next" button isn't working and I can't get to the next stage to activate the product. By now I am tearing my hair out! Can someone PLEASE help me activate my product or advise as to what I am doing wrong??????

 

Thanks