Try these steps in sequence to resolve the sign-in issues:
Verify that you are using the correct email address and password combination. If you have more than one Amazon account with the same email address but different passwords, use the corresponding password for each account.
Make sure that there are no extra spaces in your password. This can happen when you copy and paste your password from somewhere else.
Verify that you have entered the most recent Two-Step Verification code that you have received, if you are prompted to enter one. Older codes will not work. For more information, see Two-Step Verification FAQ.
To reset your Two-Step Verification follow the steps here
Clear your browser cookies and cache or try logging in with a different browser or device.
Use our Password assistance page to verify whether the email address that you are using is the one registered in our system.
If yes, change your password using the Password assistance page.
Log in to Seller Central using your new email address/password combination.
If these steps do not resolve the sign-in issue, it is likely that your email address and password combination is not associated with an active Seller Central account. If you have not deactivated your account, click on the button below for assistance:
Additional sign-in troubleshooting
I've forgotten my password.
Use our Password assistance page to change your password.
Please do not create a new Seller Central account.
Note: If you change your password and are using something other than Seller Central (for example, a SOAP server or the Amazon Merchant Transport Utility) to submit product-related and order-related data, you might need to reconfigure those services with your new password.
How do I access my company's Seller Central account?
If your company has already registered with Seller Central, your company's account administrator can set up a user account for you.
If your company has not signed up to use Seller Central, visit www.services.amazon.co.uk to find out how you can get started.
How do I get technical support for my account?
If you already have a Seller Central account, contact us using Seller Central. You must sign in first.
What happens if I do not receive my Two-Step Verification code via SMS text message on my mobile phone?
Click the "Didn't receive the code?" link on the webpage where it asks you to enter your code. This will list any backup methods that you designated when you set up your account. If you do not receive the SMS text, you can opt for a voice call to your registered mobile phone number. If you do not receive a code during registration, check that the provided phone number has no typos and includes a region code and that your mobile phone is able to receive SMS text messages.