When you're using a third-party browser on macOS such as Google Chrome, Mozilla Firefox, etc, you may need to reset permissions for your browser in macOS settings.
Solution
Remove Browser Permissions
Click on the Apple menu button and select System Preferences.
2. Click Security & Privacy and click on the Privacy tab.
3. Click Files and Folders from the left panel.
4. Then find your browser and uncheck the Downloads folder permission.
5. Now close and relaunch your browser.
With this, permissions should be reset and you should be able to upload documents from your computer to OmniPlatform.