> The File is a txt File. I copied the Signature String, but this won't work..
> "license": "<Base64 encoded license text here>"
Like the error says, the value must be a base64-encoded license string. Take your license string (I don't know if that is "Signature String", I've only ever used the free license included in SG archive package) and use your Google Fu to find how to convert it to Base64 (best to do that offline rather than use any online Web service).