What should be done to add or resize the datafile?
Nothing can be done. The XE version is limited to 11GB of data, see this link
Oracle Database Express Edition
Oracle Database Express Edition (Oracle Database XE) is an entry-level edition of Oracle Database that
is quick to download, simple to install and manage, and is free to
develop, deploy, and distribute. Oracle Database XE makes it easy to
upgrade to the other editions of Oracle without costly and complex
migrations. Oracle Database XE can be installed on any size machine
with any number of CPUs, stores up to 11 GB of user data, using up to
1 GB of memory, and using only one CPU on the host machine. Support is
provided by an online forum.
You can purchase Standard or Enterprise edition, which don't have this restriction (but you must pay for it).
You can also register in Oracle's Technology Network and use any of Oracle's product (including Oracle's Standard and Enterprice database editions from their download site) for free, but only for testing and development purposes.
See this licence for details:
License Rights and Restrictions
Oracle grants You a nonexclusive,
nontransferable, limited license to internally use the Programs,
subject to the restrictions stated in this Agreement, only for the
purpose of developing, testing, prototyping, and demonstrating Your
application and only as long as Your application has not been used for
any data processing, business, commercial, or production purposes, and
not for any other purpose.