The INSERT statement conflicted with the FOREIGN KEY constraint

hi guys, 
I have been having this problem for a couple of hours now. in my sql server, I inserted this query :
INSERT INTO USERS_AVATAR(userId, avatId) VALUES ('1', '213'); INSERT INTO USERS_AVATAR(userId, avatId) VALUES ('2', '312');

but the message shows up saying :
Msg 547, Level 16, State 0, Line 1 The INSERT statement conflicted with the FOREIGN KEY constraint "FKUSERS_AVAT731248". The conflict occurred in database "gk314", table "gk314.USERS", column 'userId'. The statement has been terminated.
Msg 547, Level 16, State 0, Line 2 The INSERT statement conflicted with the FOREIGN KEY constraint "FKUSERS_AVAT731248". The conflict occurred in database "gk314", table "gk314.USERS", column 'userId'. The statement has been terminated.

Help please!!!!