The Global Administrator is not an Azure role, it's an Entra ID role. This does not give you privileges to manage Azure subscriptions, only Entra ID resources (users, groups, etc.). You can, however, gain access to Azure subscriptions associated with the Entra ID tenant by following https://learn.microsoft.com/en-us/azure/role-based-access-control/elevate-access-global-admin?tabs=azure-portal
hth
Marcin