Why can't I log into Sengled.com with the same account that I use for my Snap app?

Sengled is working hard to unify this part of the customer experience.

Currently, the Snap app and Sengled website use separate accounts.

You may use the same email address and password for each of these services; just remember that resetting your password or changing your email address in one service will not carry over to your other Sengled accounts (at least not for the time being).