How to determine US source and foreign source from sch k-1 line 16C

You should have a schedule attached to your K-1, per the IRS instructions here: https://www.irs.gov/instructions/i1065sk1#idm139827348326128

If you don't have this you might need to request it from the business/firm that issued you the K-1. However, often you can get away without it. 

If you are getting an error, make sure you update the foreign income section for the foreign amounts entered on your K-1. To do so:

1. Open your return
2. Select the Federal Taxes tab and then click the Wages & Income tab below it  (Click I'll choose what I work on) 
3. Scroll down to the Less Common Income section 
4. Click Start/Update next to Foreign Earned Income & Exclusion
5. Next, select that the Schedule K-1 from a business reported the foreign income and complete the questions that follow.