When a pipeline uses a remote, 3rd-party repository host such as Bitbucket Cloud, the repository is configured with webooks that notify Azure Pipelines Server or TFS when code has changed and a build should be triggered. Since on-premises installations are normally protected behind a firewall, 3rd-party webhooks are unable to reach the on-premises server. As a workaround, you can use the External Git repository type which uses polling instead of webhooks to trigger a build when code has changed.
References:https://docs.microsoft.com/en-us/azure/devops/pipelines/repos/pipeline-options-for