Tropy 1.6 requires macOS 10.13 or later; we have modified the update server to stop delivering updates to older macOS versions, so if you re-install Tropy 1.5.4 from here you can continue to use that version.
Tropy 1.6 requires macOS 10.13 or later; we have modified the update server to stop delivering updates to older macOS versions, so if you re-install Tropy 1.5.4 from here you can continue to use that version.