Keychains need to be unlocked before they can be used. You can use security unlock-keychain to unlock. You can do that interactively (safer) or by specifying the password on the command line (unsafe), e.g.:
security unlock-keychain -p mySecretPassword...
Obviously, putting this into a script compromises the security of that keychain, so often people setup an individual keychain with only the signing credentials to minimize such damage.
Typically in Terminal the keychain is already unlocked by your session, since the default keychain is unlocked on login, so you don’t need to do that. However, any process not run in your session won’t have unlocked keychain even if it has you as the user (most commonly this affects ssh, but also any other process).