If you signed in with a public key, that would explain why it doesn’t work. I just removed that option from the nostr-login modal.
Discussion
Maybe because I have used up my nostr.build free privileges?
Just tried making a new account, and I just get a spinning "connecting, establishing connection to your key storage" notification.
I'm not aware of a total limit for free accounts. You need to accept the nsec.app connection request. If you're on iOS, there's special instructions https://nsec.app/ios.html. It's much easier to use a nip-07 extension though. I use nostr:npub1tnpfz60snm7ler8k8c69335n37we6690qttl88t556yzkjxhahjq3kkkpe on iOS.
No limits for free usage, just the file size.
et pour Android
https://nsec.app should work, but I haven’t tested