"at" command keeps popping an alert

You have to provide "Full Disk Access" to Terminal to prevent this kind of alert:

  1. Open System Preferences from Apple menu.
  2. In System Preferences click on Security & Privacy.
  3. In Security & Privacy, click on the Privacy tab.
  4. Select Full Disk Access from the left sidebar.
  5. Click on the Lock icon to make changes, and provide it an administrator user / password.
  6. Click the + button (below the list of apps).
  7. Add Terminal App (it can be found in Applications > Utilities).
  8. Make sure that the checkbox next to Terminal is ticked.

Screenshot of macOS System Preferences - Security & Privacy - Privacy dialog window