In my K-1 form for S corp (1120-S), Box 14 is checked, but the attached K-3 has only US. However, TurboTax does not allow me to enter US. How can this be entered?

you enter the k-1 info but skip the k-3 info.  the K-3 is for foreign reporting. what most software does is prorates everything among the shareholders.  you only mention US income reported. even if you had foreign tax credit or deduction US income has no effect.  however, there could be other parts to the k-3 that may require reporting just t that other than the foreign tax credit part Turbotax doesn't handle those tax forms. 

here's a partial summary for a k-3 from a partnership  The k-3 for partnership and S-Corp are identical

also note because of the lateness in finalizing the K-3, the IRS generally is waiving some of the reporting requirements - (not part 2 but part 2 would only apply if there were foreign taxes)

all of the following assumes you are a US citizen
the K-3 

PART:
1) other forms but they are not specified
2) all US income so there's nothing to report for form 1116
3) this is for apportionment and allocation of research and experimentation expenses for foreign tax limitation purposes but you say no FTC so this section is irrelevant
4) for preparation of form 8993 to determine section 250 deduction. but this only applies to domestic corporations and US individual shareholders of controlled foreign corporations making a section 962 election. IRC 962 election is an election to be taxed as a Corporation. And, most taxpayers would not want to elect to be treated as a corporation and then become double taxed.
9 and 10) appear to be for foreign partners.

Turbotax can't handle most of this info only section 2 when needed. if other sections apply to a taxpayer they would have to enter the numbers directly on whatever form or schedule they apply to and many of these forms are not covered in any version of Turbotax.

these include forms 8858, 5471, 8993

generally 8858, 5471 would only need to be filed if you owned through the S-Corp more than 10% of a foreign corporation or ar an officer  or director of a foreign corporation owned by the S-Corp