Question { Oracle, 35224 }

ORA-01100: database already mounted

Answer

i think "database already mounted" means that

oracle instance is associated with your current database (
which ever you have database name (name may be "xe") )

so your new database in not creating because that instance
is engaged with your older database ,

ACTION - edit the pfile ( it is the file to create new
instance parameter) so that new instance will be created

now you can write the create database statement