Advanced Hunting is not working

Global admins should have access to Advanced Hunting. Possible explanations:
1) you were told your account was a global admin, but it is not
2) you don't have a licence for a corresponding Defender product
3) there is an environmental factor (web proxy, old browser, etc) stopping you

Please check the permissions granted to you under Permissions & roles section in left pane. To access advanced hunting, you need one of the permissions mentioned here https://learn.microsoft.com/en-us/microsoft-365/security/defender/custom-roles?view=o365-worldwide#required-roles-and-permissions. If you are still not able to access advanced hunting, please create a support ticket.