As you can see, that doc indicates the URL is service/public/v2/api/policy. However, this is for Ranger 0.5, which is in HDP versions 2.3+
If you are running Ranger 0.5 as per the above, can you try that URL and let us know if it works?
EDIT: Just rereading, I realise you are probably trying to import a set of policies? Or are you just doing one? If just creating one new one, you use the statement I said above, but if you want to import a set of policies that you exported from another cluster, you do this, according to this documentation. Note the "servicesMap" distinctions and the "multipart"
Ah yes - I was getting the same behaviour, but then i realised if you are uploading a file with the -d command, you have to do @ before the file name. This command worked for me on Ranger 0.6 on HDP 2.5.0....