Start with the right checks.
The host may need to allow participants to share. A computer permission problem is a separate issue, particularly on a Mac.
What to do next.
Check whether the Share control is available. Ask the host whether participant sharing is allowed.
Choose the specific window you intend to show rather than the entire desktop when practical.
If Zoom requests a system permission, follow the relevant official instructions for your operating system. You may need to leave and reopen Zoom after the change.
Update Zoom through its official update process when you can leave the meeting safely. Retest in a private meeting before the next presentation.
Check the result.
Ask the other person to name what they see. A sharing indicator on your own screen alone does not confirm the correct content reached them.
Stop and get help if…
The host has restricted sharing, your computer is managed, or the shared image stays black. Ask for help rather than disabling security tools.