Looks like Apache Nifi is not coded for the available streaming endpoints. There is no public firehose endpoint (and there never has been one) - that is commercial and available via our data business. The sample endpoint provides a 1% sample of the firehose, and the filter endpoint provides up to 1% of the firehose depending on your chosen tracking terms. They should probably fix the code so that it does not offer the confusing unavailable option.
We can’t offer direct support for third party code so I’d recommend chatting with the Nifi project if you’d like to resolve this.