There are some rare cases where it is correct for an employer not to withhold social security or medicare tax, so Turbotax can only nag you "are you sure this is correct?", it can't force you to pay the tax.
What type of work do you do? Is it possible you are legitimately exempt?
If you are not exempt, you need to include form SS-8 with your tax return to pay the uncollected 7.65% social security and medicare tax. Since you already filed, you need to wait until your return is fully processed and any refund paid, then you can prepare an amended return. (Also, the amending function is not yet turned on in Turbotax.)
