Can't install SourceTree 2.7.6a on Mac 10.13.6

[sdronam]

I have a MacBook Pro 2017 running MacOs High Sierra Version 10.13.6. When i download the Sourcetree 2.7.6a zip file from SourceTree site and unzip it, I get a .app file. I double-clicked on the file and it says cannot open this file. I tried copying the file to Applications and then clicked in it, it gave me the same issue. I also tried to restart the system and check, it gave me the same issue. The only version that i am able to run on my machine is version 2.1 because its installation file was a .dmg file. Need help to troubleshoot this issue.

[bgannin]

Does the app update once installed?

[sdronam]

No. Even the updates fail. I installed version 2.1 using the .dmg download file. After that, whenever I try to run an update, it fails because the next version comes in a .zip file and because of that, the upgrade fails.