Good question! The feature is part of Event Ticket Plus. Do you have that and Event Tickets both installed and active on your site?
If they are installed and active, will you please head to Events > Settings > Help, copy the system information at the bottom of the page and paste it here for me as a private reply? That will allow me to check your installed versions.
Ah yes, I see you have Event Tickets Plus version 4.0.5 installed and the feature for Global Stock was released in 4.1. I’m afraid there is no snippet to simply enable the feature, but upgrading to the latest versions of Event Tickets Plus and Event Tickets will ensure that the feature is enabled.
Does this help answer your question? Please let me know.