"Relation does not exist" error, only with libpq - c

Question

I'm trying to run this query to insert new row to Users table from my C code using PQexec() (libpq)
INSERT INTO Users
VALUES ((
SELECT MIN(s.id)
FROM generate_series(1,(
SELECT GREATEST(MAX(Id) + 1,1) FROM Users
)) AS s(id)
WHERE NOT EXISTS (SELECT 1 FROM Users WHERE Id = s.id))
, 'Tester', 27)
RETURNING Id;
It performs what i need when i run it in psql terminal, but from C it returns
Error executing query: ERROR: relation "users" does not exist
I checked connection status and it succeeded, using the same user i connect to from terminal. How come it can't find the users table?
EDIT: adding C-code
Connection:
sprintf(connect_param,"host=address dbname=%s user=%s password=%s",
USERNAME, USERNAME, PASSWORD);
conn = PQconnectdb(connect_param);
Query:
sprintf(cmd, "INSERT INTO Users "
"VALUES (( "
"SELECT MIN(s.id) "
"FROM generate_series(1,( "
"SELECT GREATEST(MAX(Id) + 1,1) FROM Users "
" )) AS s(id) "
"WHERE NOT EXISTS (SELECT 1 FROM Users WHERE Id = s.id)) "
" , \'%s\', %d) "
"RETURNING Id;", Name, Age);
res = PQexec(conn,cmd);

Answer 1

I am going for deduction:
Error executing query: ERROR: relation "users" does not exist
This kind of error is throw when the databases doesn't find the table(view, or wathelse can pass through a SELECT, he gives the name of "relation") .So your code looks fine, but a sub-set of reasons can be:
the table users doesn't exists. Some spelling mistake
you perform the query in the wrong database (where this table is not defined)
you perform the query in the wrong server (as above)
you perform the query in the wrong schema (as above)
The string get truncate from the sprintf.
and similar. The connection works very well since you get an answer from the database

Answer 2

your problem of library link to compilation.
gcc -I/usr/include/postgresql/ -L/usr/lib/postgresql/8.3/lib/ -lpq