In order to build a xcworkspace or a xcproject, a shared Xcode scheme is required. Xcode schemes are saved locally so that Xcode has access to them, but by default they are not shared with others and they are not included in source control.
To share a scheme, click on "Manage schemes" in Xcode and make sure that the scheme used for building your xcproject/xcworkspace is marked as shared. The scheme's container needs to be set to the Xcode project or workspace you want to build. Then make sure the added xcscheme file is added to source control and pushed to your remote repository - which App Center accesses.