why im getting this error?

[cipriano macuja]

error: failed to push some refs to 'https://bitbucket.org/knox_macuja/git-test.git'
hint: Updates were rejected because the remote contains work that you do
hint: not have locally. This is usually caused by another repository pushing
hint: to the same ref. You may want to first integrate the remote changes
hint: (e.g., 'git pull ...') before pushing again.
hint: See the 'Note about fast-forwards' in 'git push --help' for details.

[Nic Brough -Adaptavist-]

Welcome to the Atlassian Community!

This happens when you create a new git repository the same as an existing one, but with a README file.  The message is telling you that you need to merge rather than try to start again.

Do what it says - a git pull will get everything in line for your next push.