"Where do I enter the sale of business property in TurboTax Business 2018?"
If the property had been associated with (or used as) a rental or royalty property, then start in the Rentals and Royalties section of the program (see screenshot).
If otherwise, then you will have to prepare that part of the return using Forms Mode. All of the relevant forms are available, but there is no Step-by-Step section that guides users through the sale of a business asset that was not used in connection with, or as, a rental or royalty property in the 1041 module of the program.
