@Looneybin1978 That's not how things are supposed to work: the accounts should connect automatically when you try to launch Sims 4, no product code needed. Since that's being blocked though, let's try another way. Go to ea.com in a different browser or in a private window so that you're not already signed in. Instead of entering your Origin username or email, click the Steam logo at the bottom of the page, and enter the username and password of the Steam account that owns Sims 4. Let me know what happens.
If you get signed in properly, go back to Account Settings as before, and take note of your EA username and email address. You don't need to tell me what it is; just sign into that account within Origin and then see whether Steam lets you launch Sims 4.
If you can't sign in to ea.com with your Steam credentials, please post a screenshot of the error you get.