I have a K-1 from a Form 1120S! Where do I enter Line 17, Code AC?

The first step of this is google-able.  Per irs.gov form instructions:

Code AC. Gross receipts for section 448(c) Use the gross receipts amount to figure the business interest expense you can deduct, if applicable. See section 163(j) and the Instructions for Form 8990 for details.

So you'll have to see if this applies to your client.  The items further down on the K-1 don't always fit neatly into K-1 input boxes on the personal returns, so it takes extra due diligence form you to 1) ask you client for relevant facts and circumstances (that they will likely be clueless about), and 2) deep dive into further research.

Good luck!

I googled on irs.gov!  I researched starting with K-1 instructions! That led me to Form 8990! Form 8990 led me back to K-1 instructions! I researched this for  well over  2 hours, it should not be this difficult! I contacted the entity that issued K-1! They are going to check with CPA firm who prepared the Form 1120S!  Thank you for your response!