Form 1099-B has a section for "Long-Term transactions for which basis is available but not reported to the IRS". What does this mean?

under the tax laws, brokers are required to report tax basis, when available,  for all sale/closing transactions for securities acquired after 2013. proceeds must be reported regardless of date.   in some cases the broker doesn't have the tax basis or is not required to report it - so those are of the type you're asking about. 

a few examples: inherited securities, securities transferred from one brokerage to another, certain investments like publicly traded partnerships where the broker gets no updates as to changes in tax basis. 

Since the brokerage is not reporting the basis to the IRS, you have to provide the basis on your tax return. If it's an investment that you purchased yourself, your basis is what you paid for it. When TurboTax asks for the sale category, you will select "Box E - Long term noncovered."