GitHub have recently closed down some of the older security protocols and old versions of Git will no longer work. SourceTree comes bundled with a pretty old version of Git (1.9.5). So if you want to work with GitHub you should download a newer version of Git from here (https://git-scm.com/download/). Then switch SourceTree to use that instead of its embedded git, which you do by going to Tools > Options > Git > Use System Git
Or you could just install GitKraken or go command line
