Although this documentation says that "Key words and unquoted identifiers are case insensitive." it is not possible to use table or column names in upper case.
If the table or column name was in upper case it is necessary use quotes to Postgres accept. Exactly the opposite case showed at documentation.
For example:
CREATE TABLE CLIENT(ID INTEGER, NAME TEXT);
SELECT * FROM CLIENT; -- an error will be launched
SELECT * FROM "CLIENT"; -- works
Are you sure about the CREATE TABLE statement you used? Which version and OS?
Because this is what I get, which matches exactly the documented behaviour (9.5.4, Ubuntu):
x=# CREATE TABLE CLIENT(ID INTEGER, NAME TEXT); CREATE TABLE x=# select * from client ; id | name ----+------ (0 rows)
x=# select * from "CLIENT" ; ERROR: relation "CLIENT" does not exist LINE 1: select * from "CLIENT" ; ^ x=#
