I've got several tables made but i need to link two tables together with pk & fk.

The two tables are:

CREATE TABLE Game_Play
(GameNo SMALLINT,
DatePlayed DATE,
MethodologyType VARCHAR(50),
No_of_SDLC_Iterations SMALLINT,
No_of_Players SMALLINT,
CONSTRAINT gameplay_pk PRIMARY KEY (GameNo));

CREATE TABLE Player
(PlayerID VARCHAR(5),
PlayerName VARCHAR(50),
CONSTRAINT player_pk PRIMARY KEY (PlayerID));

I am trying to make playerid from the player table link to a field in the game_play table called playerid_winner as a foreign key.

i am using oracle sql*plus.

i have tried to add a new field and create the fk with the below

ALTER TABLE GAME_PLAY ADD CONSTRAINT PLAYERID_WINNER
FOREIGN KEY (PlayerID) REFERENCES PLAYER(PlayerID);

and i have this error appear

FOREIGN KEY (player_pk) REFERENCES PLAYER(PLAYER_PK)
*
ERROR at line 2:
ORA-00904: "PLAYER_PK": invalid identifier

Any ideas?