To make the clone or svn2git process work I am pretty sure you are stuck with one of the two solutions of:
In svnserve.conf setting:
anon-access = read
To: anon-access = none
Or if it is served via Apache using the Apache SVN module in the Authz file linked in the comment making the change:
[/]
* =
To:
[/]
* = r
All of that said:
Given that the above is a non-solution to your problem if you are unable to have the server configured properly to allow you to do this you could see if you have any better luck with the svnsync command:
You could then use your local SVN to create your git repo. Assuming your intent is to be able to push changes back to the original SVN source, I think this will not require the needed configuration discussed above for the initial clone, and you would be able to get it working from this point... This entire recommendation is based on a chain of "maybes", so hopefully someone else has a better solution if you really need to do this and cannot have the configuration of the server altered.