Well. the output of the python hmac is a byte array and the output of the LV hmac is a hex string so I expect you have to convert the LV hmac output into a byte array before passing it to the encode like this:
Generate Signature.vi
By the way. None of your sources (message or key) are base64 encoded so you may run into errors when trying to b64decode normal text.
At a guess I expect it's because you are decoding with UTF-8 in your python.
I checked the HMAC library you are using and it produces the correct hashes for test vectors.