Yeah, it seems like Intuit hasn't updated its software very well for the Tax Cuts and Jobs Act, including the new rules for Section 179.

*IF* it qualifies, I would ignore the diagnostic and file.

However, Section 179 only applies to property used in a "Trade or Business", so you would first need to make the determination if the rental property rises to the level of a "Trade or Business".  The diagnostic should probably be changed to remind you of that requirement, rather than saying it is not allowed.