Short Answer 
To ensure proper OAuth functionality in Sourcetree, verify your Bitbucket account credentials in Sourcetree, and then check and update the OAuth settings in your Bitbucket account. Finally, input the updated credentials in Sourcetree and test the connection to resolve any potential errors.
Step 1: Verify OAuth Credentials in Sourcetree
Start by opening Sourcetree and navigating to the account settings. Here, ensure that you are logged into your Bitbucket account properly. Check if Sourcetree has the correct permissions, as this is crucial for OAuth functionality. If there are any missing credentials, input them as needed.
Step 2: Update OAuth Settings in Bitbucket
Next, go to the settings in your Bitbucket account. Look for the OAuth consumer section to ensure the Client ID and Secret are correctly set up. If necessary, create a new OAuth consumer. Remember to grant all necessary permissions to allow Sourcetree proper access.
Step 3: Finalize Settings and Test
Once the credentials have been updated, return to Sourcetree and enter the newly generated Client ID and Secret in the account settings. After saving these changes, attempt the pull operation again to see if the error has been resolved. Ensuring these settings are correct is essential for successful communication between Sourcetree and Bitbucket.