Hi Sangwan,
was mentioned in my first answer, without a copy of the error you get, maybe more information about the database system you actually use and a copy of your configuration there's not much help anyone can give you.
A pretty common problem is that the tables you already created, are restricted to be seen by another user than liquibase uses to connect to the database.
So try to connect to the database with the same user as liquibase does and select from the databasechangelog table works.
Double check that the structure you created is identical to what liquibase would create in a fresh instance.
Also make sure the user liquibase uses has the rights to create alter and maybe drop database structures.
kind regards
Daniel