How to make "git push/pull" work in non-clone repo?

Hi,

I create a repo X with git init. After several commit in X, I clone a
bare repo Y from X.
I try to continue work in X, and push to/pull from Y. The command git
push and git pull fails. I see the error message but I do not know
which git command can fix that problem.
Then I clone Z from Y. git push/pull works in Z.
How to make "git push/pull" the same in X as in Z?

Thanks,
Hong-Ming