Hi @manchestermania,
you will not be able to use the same purchase code in more than one website.
So, you have to deactivate the license from the old website then you will be able to use the license for the new website.
for solving purchase code or item license issue will need to get support from the item author. They (author) are the right team to help. you can contact item author through item comments page or author profile page or best option is through item support page.
How to contact an author to get support or for any technical query:
Thanks