Turbo tax does not handle this code directly. box 20 is for informational purposes. some have to be entered elsewhere others can be ignored. this can not 453A(c) is interest (really an added tax) on deferred tax from installment sale over $150,000. the amount goes on schedule 2 line 8 and is label as 453A(c).
in step-by-step mode, it is under other tax situations
then misc tax forms
then additional tax and repayments
then (2nd screen) other taxes
where you should see a box for this
not sure which version of turbo tax you're using so these directions may not be correct.