I think it depends what you mean by Shopify amazon link and what's meant by a pro account.
I'm guessing it means connecting the Amazon fulfillment service to Amazon seller central professional account. If that's the case and you already upgraded your seller central account, I would delete the amazon connection you have, clear your browser cache, and try connecting again.
If it's a Shopify pro account that's required, maybe you're on a new store and need to upgrade to a paid plan (this is common for apps).
If it's neither, as a general rule I recommend this process for debugging and getting solutions:
1) State the problem clearly
2) Give steps to reproduce the problem
3) Include screenshot if it helps give more information
That will help someone visualize the problem and try to reproduce so they can help you solve it.