Quick Summary
Error 91 on macOS usually indicates a problem with application access to a required resource or file, potentially due to permissions or corruption. This can occur during application launch, a specific function, or when attempting to save or open a document.
Common Causes
- Insufficient Permissions: The application may not have the necessary permissions to access a specific file or folder it needs to operate correctly. This is a common cause when the affected files are present in a protected area or have been manually modified.
- Corrupted Application Files: One or more files within the application package may have become corrupted, preventing the application from functioning as intended. This can be caused by disk errors, incomplete updates, or sudden power outages.
- Conflicting Software: Another application or system process may be interfering with the application's ability to access needed resources. This is especially true for security software or other apps that actively monitor system access.
- Operating System Issues: Underlying macOS system errors or corruption can sometimes manifest as application-specific errors. This is less common, but it's something to consider after simpler solutions have been ruled out.
Step-by-Step Fixes
Method 1: Verify File Permissions
Step 1: Locate the file or folder the application is trying to access (if known). If not, check the application's settings for recent files/folders it may have used.
Step 2: Right-click the file/folder and select 'Get Info'.
Step 3: In the 'Sharing & Permissions' section, ensure your user account has 'Read & Write' access. If not, unlock the padlock icon (if present using your admin password), change the permissions to 'Read & Write', and then lock the padlock again.
Step 4: If your user account is not listed, click the '+' button to add it and then configure the permissions.
Method 2: Reinstall the Application
Step 1: Quit the application completely if it is running.
Step 2: Drag the application icon from the 'Applications' folder to the 'Trash'.
Step 3: Empty the 'Trash'.
Step 4: Download the latest version of the application from the official website or the Mac App Store.
Step 5: Follow the on-screen instructions to install the application.
Method 3: Run Disk Utility First Aid
Step 1: Open 'Disk Utility' (located in /Applications/Utilities).
Step 2: Select your startup disk (usually named 'Macintosh HD').
Step 3: Click the 'First Aid' button.
Step 4: Click 'Run' to start the disk repair process. Let it complete uninterrupted. This may take a significant amount of time depending on the disk size and severity of any errors.
Method 4: Check for Conflicting Software
Step 1: Identify any recently installed applications or system utilities, especially those related to security, system monitoring, or file management.
Step 2: Temporarily disable or uninstall these applications.
Step 3: Relaunch the application that was causing the 'Error 91' to see if the problem is resolved. If the error disappears, you've identified a conflicting application. You can then try re-installing conflicting programs (or finding an alternative) one by one to determine the exact cause.