Code AC in line 17 of an S corporation schedule k-1 refers an exemption to the limitation on the deduction of business interest if the company's average annual gross receipts for the past three taxable years was not more than $25 million dollars. You would need to enter the code AC in your box 17 entry on the K-1 schedule and the corresponding dollar amount of gross receipts. If it is missing from your K-1 schedule, it may mean the company no longer qualifies for the exemption, so you should check with the individual that prepared the K-1 schedule if you are unsure.
From the IRS:
You can edit your K-1 entry by first going to the federal section of TurboTax, then Wages and Income, then Business Investments and Estate/Trust Income, then Schedule K-1. When you update that section, you will come to a page that says Check boxes that have an amount or are checked on the form, check the box 17 option and when you continue you will come to the Box 17 entry screen where you can choose "AC" in the left drop down menu and enter the amount in the column next to it.