We are unable to reproduce this on our end. I see that loading https://sellercentral.amazon.com/ in the browser takes me to the Seller Central login page. If you are already logged in, it should take you to the App Console for whichever Amazon product you have onboarded with. If you have onboarded with multiple products, you should be able to navigate between those using the marketplace switcher on the top.
If you still issues, please send us a screenshot of the experience, along with any other details that may help, including the browser you are testing on.