I had the same problem approx 1 week ago
I updated to the Cloudsmith etc etc, and all was good
Until this afternoon.
E: Failed to fetch https://dl.cloudsmith.io/public/balena/etcher/deb/linuxmint/dists/ulyssa/InRelease 402 Payment Required [IP: 126.96.36.199 443]
E: The repository ‘https://dl.cloudsmith.io/public/balena/etcher/deb/linuxmint ulyssa InRelease’ is no longer signed.
N: Updating from such a repository can’t be done securely, and is therefore disabled by default.
Do I need to be using another app ?