I already filed my taxes, but Turbotax did not allow me to enter my interest on my mortgage statement? Why?

Most likely, the program concluded that your standard deduction was a better choice then itemizing (entering your mortgage interest and SALT) given your financial situation.  

Under the new tax law, the SALT deduction (state & local tax, property tax, vehicle registration, sales tax) is capped at $10,000 ($5,000 if married filing separately). Prior to that, there was no cap. Taxpayers must still itemize to get this deduction – that part hasn't changed.

As a result of these and other tax law changes, our estimate is that nearly 90% of tax filers will now be taking the higher standard deduction, up from around 70% last year. And if you're in the 90% group, you won't see a change in your refund after entering your mortgage interest and property taxes.

The 2018 standard deduction is nearly double the 2017 amount:

$12,000 for Single

Add $1,600 if 65 or older

Add $1,600 if blind

$18,000 for Head of Household

Add $1,600 if 65 or older

Add $1,600 if blind

$24,000 for Married Filing Jointly or Surviving Spouses

Add $1,300 for each spouse 65 or older

Add $1,300 for each blind spouse

$12,000 for Married Filing Separately

Add $1,300 if 65 or older

Add $1,300 if blind