This would be the right time to request a meeting with your real estate agent.
It is for is purpose most agents attend the inspection. Should something be found that need correction. I always ask, "What does a fix look llke?" If the resonse is, "Two inches needs to be removed from this 1/2 PVC drain pipe..." I can fix it faster that the inspector can write it up. One inspector gone goofy was going to write up a plug not fully seated in the receptable until my toe pushed it into place.
Your agent very likely compared the list of repairs you requested and the seller agreed to and verified via receipts, personal visit, or during the final walk through with you. You ALSO should have been looking to confirm such requested repairs were completed and confirming no new issues such as under sink leaks has occurred.
If it is factual that the repair was agreed to by the owner and was not fixed (be aware, the nature of the problem may have been a blockage causing slow drainage requiring a snake. This could have been completed by the current owner and there would be no evidence or abiliy to visually confirm.) We do not have sufficient data to know.
If your agent confirms the repair was agreed to and not completed, your agent can push the request to the former owner, if able to find, using correct protocol. Your next options is balancing the open ended cost of an attorney to the fixed cost of doing the repair as you intend. Let's not overlook the conditions of the home sale such as "As-Is'.
A sign off on the final walkthrough means you found everything satisfactory. An attorney would LOVE to hear from you, but whether that action is truly in your best interest is up to you to decide.