I am unable to select the Server-to-Server OAuth app type in the Zoom Marketplace under Develop → Build App.
The account I am using has the Admin role.
Have your account owner set up a custom role that grants that (Zoom for developers; Server-to-server OAuth app) permission, then assign you that role. Also include anything else that your application will need to do, since your application will inherit a subset of those permissions based on the scopes you grant it, which you can only grant if your account can do the same thing manually.