Hi there! I understand you're having trouble equipping turrets in the Ammo Fever app on your Mac with OS X 10.15.7. To help you better, could you tell me a bit more about what happens when you try to equip a turret? Do you see any error messages, or does the option just not respond? Also, do you have the turrets in your inventory or storage already?

Meanwhile, here are some initial troubleshooting steps we can try:

1. Check Turret Availability: Make sure the turret is actually in your inventory or base storage. Sometimes turrets can be destroyed in missions and appear as "missing key" or unusable items.

2. Restart the App: Close Ammo Fever completely and reopen it. This often resolves minor glitches that prevent equipment actions.

3. Update the App: Ensure your app is updated to the latest version via the App Store. Sometimes bugs fixed in updates affect turret functions.

4. Inspect Turret Compatibility: Verify that the turret you want to equip is compatible with the slot or area you’re trying to place it. Some turrets or guns may have restrictions.

5. Check for In-Game Requirements: Some turrets may require certain resources or conditions like ammo availability or power to be equipped.

If these don't help, we can try some deeper steps:

- Verify your game data is not corrupted by reinstalling Ammo Fever (make sure your progress is backed up or linked to an account if possible).

- Review your inventory and turret items for any status like "Missing Key" which can prevent equipping (this issue has been noted in other users).

Could you please try these and let me know specifically where you get stuck or if any message pops up? That will help me guide you further.

Are you able to see the turrets in your inventory/storage list right now?