Hi Fion, welcome to the Atlassian Community!
The full list of Sourcetree versions available for download is here: https://www.sourcetreeapp.com/download-archives
In the upper right corner of that screen, you'll see a blue "Download free" button which will get you the most recent version. I believe even the most recent 4.x release should run on MacOS 10.13. Previous versions of Sourcetree are all listed on that page too.
I also can confirm such issue, MacOS 10.13.6. will not allow to install ST v4.0 - because it requires OSX 10.14 :(. no info logs just well known console output
AEGetDescData of non-desc type 'reco' not a good idea
Any hint how to proceed without upgrade to OSX 10.14?
it is blocker on OSX 10.13.6
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