Query in SQL statement

Am trying to port a mysql statement to postgres.

Please help me in finding the error in this,

CREATE SEQUENCE ai_id;
CREATE TABLE badusers (
id int DEFAULT nextval('ai_id') NOT NULL,
UserName varchar(30),
Date datetime DEFAULT '0000-00-00 00:00:00' NOT NULL,
Reason varchar(200),
Admin varchar(30) DEFAULT '-',
PRIMARY KEY (id),
KEY UserName (UserName),
KEY Date (Date)
);

Am always getting foll. Errors,

ERROR: relation "ai_id" already exists
ERROR: syntax error at or near "(" at character 240

Thanks,
Rajesh R