There are two reasons why the system doesn't give you the option to select your subscription when trying to add an Organisation to it:
The subscription you bought is not for the correct product (you have a Xero-connected Organisation but purchased a subscription for QuickBooks Online).
In this case, you need to contact Sales with a request to change the product for your subscription.
The subscription you bought is not for the right plan (you have stand-alone workflows in your Organisation but didn't purchase a Premium plan subscription).
In this case, you need to upgrade your subscription and will be charged a pro-rata upgrade fee.