Where do i enter the property taxes paid by a not fair price rental?

The fact you're renting below FMRV doesn't matter. Property taxes are entered on the SCH E in the rental expenses section. There's a box labeled Real Estate Taxes. In the desktop version it's on the 2nd "Expenses" screen.

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@anudo44 wrote:
I have a rental property in Puerto Rico that I am renting to my sister 53% of the fair price of the rental.  

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When you are renting to a family member described in Section 267, then per Section 280A every day of rental is a day of personal use unless you charge full fair rental value and the family member uses the property as a principal residence.

Otherwise, and in your case, you would report property taxes, and any mortgage interest, on Schedule A (if you itemize) subject to limitations and the rent you receive as miscellaneous income.