Sometimes when the Donation Store plugin tries to make a request to your webstores API, the request can be blocked. This tends to be because of SSL issues. If you are experiencing this problem, try to make a GET request to the API endpoint with the HTTP Secret-Key header set to that of the server. If it doesn't return JSON, it's more than likely an SSL issue.
Check out our developer documentation or our documentation relevant to customising, setting up and using Donation Store.