The problem here is your SVN repo was made with an FS format too high a version for UE4. Different versions of SVN map to different versions of its internal file system (“FS format”).
These mappings are described here.
Unreal expects an FS Format between 1 and 7, but your version of SVN has version 8, which means you are using SVN version 1.10 or higher. You need 1.9 or lower to have an FS Format of 7 or lower.
So what you will have to do is make a new repo with a version of SVN no higher than 1.9. You can do this by either creating the repo with a version compatibility option or by uninstalling SVN and installing an older version (the former is much easier).
If you are using a GUI client like Tortoise you may not have any option to set the compatibility version when you create a repo, but doing it with the command line is pretty easy:
In Explorer navigate to the folder where you want to put your repo. You don’t need an empty folder to “turn into” the repo like you can do with Tortoise.
Shift + Right click in the folder and you can see an option like “Open command line here” or “Open Powershell here”, do that. Then type the following:
svnadmin create --compatible-version 1.9 --fs-type fsfs MyNewRepository
That will make your repo. Then you can remake your checkout as normal. In the UE4 Editor near the top left there’s options for connecting to version control, and in that dialogue you just put in your repo’s URL (you can see it in the repo browser at the top).