You cannot add the XS application to the SAP standard package sap.hana.xs. Please add your XS application to the package which is named like your HANA instance (it should be created automatically when you set up the HANA instance). This package is located under the package which has the same name as your trial user
I named my HANA instance "myhana". So I got the following package structure:
- <user>trial
- myhana
Under myhana I can create further packages, XS applications ...
Regards,
Florian