I failed to pay it forward.
Why didn't you pay it forward?
It's not a big deal. I'll do it next time.
If someone does something good for you, you should do something good for someone else since people expect that. By paying it forward, it makes everyone's life better. If you don't do it, people will be less inclined to help you in the future.
But it's free and I have no money to pay for it.
Paying it forward is a nice thing to do. But if you can't afford it, that's okay. When you do have the time or money to do someone a favor in the future, you should do it.