why is the 1120-c form not available on Turbo tax home and business?
A Form 1120 is a Corporate tax return and is not supported by the TurboTax personal editions.
To complete and file a Form 1120 for a C Corporation you would need to purchase the TurboTax Business Edition which can only be installed on a Windows based personal computer - https://turbotax.intuit.com/small-business-taxes/cd-download/