Running Sourcetree 2.7.6 on macOS 10.12.6 Sierra.
Choosing "Check for Updates..." menu item results in "Update Error!" dialog with "An error occurred in retrieving update information. Please try again later." descriptive text:

This should be fixed.
If indeed Sourcetree 2.7.6 is the latest that can run on macOS 10.12.6 Sierra, then this message should instead say "No Update Available" with "You're already running the latest version of Sourcetree for your system." or some such.
If there *is* a newer version of Sourcetree that runs on 10.12.6 Sierra, then this Check for Updates functionality should find it & offer to install it.