A cloud-based service included in Microsoft 365, delivering scalable messaging and collaboration features with simplified management and automatic updates.
If you want to manage organizational relationships, the correct tab is called Organization Sharing and can be found under the EAC > Organization > Sharing or via this direct link: https://admin.exchange.microsoft.com/#/organizationsharing
Alternatively, you can use PowerShell and the set of *-OrganizationRelationship cmdlets, i.e. Get-OrganizationRelationship to list any existing ones.
Keep in mind that if you have just assigned the Global admin role, it might take a while before the access is reflected in the EAC or PowerShell. It's best to open the EAC via a new private session window, or establish a new session via PowerShell.