What should I do if I received a 1099-NEC instead of a 1099-MISC for rent and the commercial tenant refuses to send a correct form?

You can enter a 1099-NEC instead of a 1099-Misc.  TurboTax asks you which you are using.  They are treated the same.  I entered a 1099-NEC this year and a 1099-Misc last year for the same payer.

You could report the 1099-NEC in the business income section of TurboTax then an offsetting deduction for the same amount with description "Rental Income - Reported on Sch E".  That would require you to upgrade from TurboTax Premier to Self-Employed but will at least report the 1099-NEC where the IRS is going to look for it.

Another option is to purchase the desktop product "Home and Business" to complete your return. Here is link to purchase on Amazon.  You can also download your 2020 return, if your started, from Tax Tools in the left hand margin then open in the desktop product and follow the same process as with the TurboTax Online.  This option might be a little less costly for you.

Or you can just report the 1099-NEC as rents.  The income would be reported on your return but you might get a letter a year from now having to explain its in Rents and not Business income.