I can't sign in when I want to make in app purchases. But I can sign in when I'm downloading a new app. Please help.

Been experiencing the problem for a few days. I can't buy in app purchases. Tried signing out from my App Store so they would prompt me to sign in in the app but they told me my Apple ID or password is wrong. I thought my account was locked so I tried buying an app and it could work!! PLEASE HELP );