I'm assuming you are using the mpv key bindings?

Follow these instructions:

• Run these commands in Terminal to create the Lua script:

mkdir -p  ~/.config/mpv/scripts/
cat >> ~/.config/mpv/scripts/toggle-console.lua << 'EOF'
local open = false

mp.add_key_binding(nil, 'toggle-console', function ()
    mp.commandv('script-message-to', 'console', open and 'disable' or 'enable')
    open = not open
end)
EOF

• Start IINA
• Click on Settings… under the IINA menu
• The settings panel appears
• On the left side of the panel click on Advanced
• Slide the Enable advanced settings toggle button to be on (blue)
• Enable Use config directory at the bottom of the panel
• Confirm the settings panel looks like the screenshot below
• Click on Key Bindings
• Duplicate the mpv Default bindings
• Select the binding for ` and click on - to delete that binding
• Click on + and add a new binding for ` to script-binding toggle-console
• Restart IINA to activate the new settings

With that configuration I was able to toggle the console.

I don't understand why they made this so that you have to use a script just to be able to toggle the console.