So I tried this - but instead of starting my roses, I am now growing potato plants Really!! What did I do wrong?

Welcome to ATP, grammasal.

This is one of those gardening myths that keeps being repeated in spite of all the evidence to the contrary. Rose cuttings have no chance of survival when they have to compete with a potato for nutrients, sun, and moisture. I'm sure it's a good way to transport a rose cutting and perhaps to ship one, but it's not a good way to plant one.