There is a possibility that you are logged in to a d account that is different from the "d account linked with the SPWN ID you want to use".
Please reload your browser and try the following.
1. Select "Login with another ID" from here and log in with the d account you want to link with spwn.
2. Log out from the following URL
Click here for Shintaikan-live
Click here for d anime store
Click here for Ultraman
3. From the login screen, click [Login with d account]