Return of goods not accepted

I bought a milk formula S -26 at pic n pay in van riebeeck on the 1st of March 2016, when arrived at home I realised that I bought a wrong formula, the I went back on the 2nd of March 2016 to change it, I was told that they wont take it back, but the formula is sealed I never opened it, I fail to understand why they don't want to take it back, I use to have respect for pic n pay, but after this I am very disappointed in them.