If it doesn’t work, try running an svn info request on Terminal:
/Path/To/UE4/Engine/Binaries/ThirdParty/svn/Mac/bin/svn info --username=your_username https://path/to/repository
Replacing /Path/To/UE4 with actual path to your UE4 Engine directory, your_username with your SVN login & path/to/repository with actual path to SVN repo.
This should prompt you to enter your password, or authorise SVN to access keychain for it, in which case click Always Allow. If you’ve already denied svn access to Keychain, then you’ll need to input your password each time you connect via UE4.