Short Answer 
To resolve the ‘fatal: Bitbucket DC OAuth Client ID must be defined’ error in Sourcetree, first ensure you’re logged into your Bitbucket account and confirm that Sourcetree has the necessary permissions. Next, update your OAuth credentials by obtaining the Client ID and Secret from Bitbucket and configuring these in Sourcetree’s account settings, which should allow for successful repository access.
Step 1: Navigate to Account Settings
Start by opening Sourcetree and accessing the account settings. Ensure that you are logged in to your Bitbucket account within Sourcetree. It’s important that Sourcetree has the required permissions to access your Bitbucket repositories.
Step 2: Update OAuth Credentials
If you’re experiencing the ‘fatal: Bitbucket DC OAuth Client ID must be defined’ error, check if the OAuth credentials are set correctly. You can do this by following these steps:
- Open the Bitbucket account settings in a browser.
- Create or find your existing OAuth consumer.
- Copy the Client ID and Secret from Bitbucket.
Step 3: Configure Sourcetree with New Credentials
After obtaining the correct OAuth details, go back to Sourcetree and input the Client ID and Secret in the account settings. Following this update, attempt to pull from your repository again. This should resolve the error and enable secure communication between Sourcetree and Bitbucket Data Center.