Under Repository --> Repository Settings, the Security tab is always disabled.

I searched for this problem and found a suggestion to set my GPG program under Sourcetree --> Preferences on the Advanced tab, but there are a couple of issues:

1. The path currently listed, ../Cellar/gnupg/2.2.9/bin, is in fact valid (assuming Sourcetree is looking in the correct path to use that relative path).
2. Browsing for another path does not permit you to select Mac system folders (/usr, /opt, etc.). So I'm unable to point this to my current installation at /usr/local/bin.