While registering or adding a channel to your account with TubeBuddy sometimes the channel you're trying to access does not show up on the Google Account selection screen. If this occurs, it's likely because you're currently logged into a Google account that doesn't have permission to provide access to that channel.
This is a common issue, and many people may not realize they're logged into an alternate Google account. While they may have access to all their regular logins and even their YouTube Studio Dashboard, the alternate account doesn't have permission to provide access to a third-party service like TubeBuddy.
To resolve this issue, try each login option or other account login options you may have until you find the one that provides access to that channel for TubeBuddy.