Yevhenii Kurtov I think you are gettin mixed up.
you need to provide credentials to your repository in order to access it.
you can either:
1. provide them in the job configurations (and place the Jenkinsfile inside the scm), in which case you can simply call on 'scm checkout' to checkout the entire code. Jenkins then will use the credentials you used to access the Jenkinsfile in order to checkout that same repo.
2. provide them inside the checkout call, as you first did, in case you are writing your pipeline code inside Jenkins (so you never configured the scm in the job configurations) or in case you wish to checkout a different repo than the one you used to store the Jenkinsfile.
either way, you will still need to provide the notifier plugin with the credentials to mark the build status. you configure it inside the Jenkins Configure System (inside Manage Jenkins, not in the specific job). you will need to produce a dedicated OAuth credentials.
this and more you can find in the very good README in github: https://github.com/jenkinsci/bitbucket-build-status-notifier-plugin
and again - this is getting much more general than the discussed issue. you should raise those questions in Slack, or if you want you can Telegram me and I wouldn't mind helping you out as far as I can.