Not being too familiar with Obj-C or Swift myself, this may not be the right solution, but there's two things you may want to try:
1. Make sure that you're actually using the correct blog name. Your sample use "SamplePost", which I'm assuming is not in your actual code -- but wanted to confirm this as well.
2. Try checking the response code instead of relying only on `error`. Any http response code in the 400-599 range means that the request has failed. You should get a 201 response on success in creating a new post.
If neither of these approaches works, can you please open an issue directly on TMTumblrSDK
instead? This mailing list is more general purpose, but any client-specific questions can probably be better answered on the github project page itself.