Can I sue WalMart for Defamation At Work?

I work for WalMart as a Cashier and many times I work in the area where they sell Tabacco items. WalMart has posted that if I the Cashier determines that a customer is under 41 and/or under 21 years of age I can card them. So on Monday 3/23/2017 a secret shopper came through my line and I did not ask for his I.D. and was therefore reported to management. To make a long story short I was written up for not carding this person which I do not remember what happened that night. This is my first write up I have been employed with WalMart for a year and a half, I often cover Liquor and the Tabacco registers giving breaks or what not and this sign is posted in both area. My question can I bring suit to WalMart for Defamation statements resulting in this situation. I was following there rules. When I mention this to my supervisor as she was typing up the write up she told me she knows this is a grey and black area. What is your take on this?