People often get confused with the term private posting key, it's not the steemit generated password that we use to login.
Let us look at how to get your private posting key.
STEP 1: on the top right click the drop down.
STEP 2: Select the wallet option.
STEP 3: Go to permissions over there.
Here you'll see a series of passwords and note down the first one that starts with "S".
That's your private posting key.