manage third party tax site my supplier not available in drop down list

When I try to enter a third party tax payer site for a UK supplier - its not available to add the party number. I see its a tax party tax profile that already exists but when I'm adding the site the number is not populating with anything. I have set this up once for a US supplier and it worked fine. I have set our UK BU tax Application options and defaulting to supplier site same as I did for US. What setup am i missing so I can add a third party tax profile? I am trying to