"no matching unique or primary key for this column-list". The primary key does exist though - sql

Question

So i'm practicing some sql coding for a test and I can't get a foreign key to reference a primary key.
Here's the table that doesn't work:
CREATE TABLE ASSIGNMENT(
ASSIGN_ID NUMBER(2) NOT NULL,
START_DATE DATE,
END_DATE DATE,
BUDGET NUMBER (10,2),
MANAGER_ID NUMBER(2),
PRIMARY KEY (ASSIGN_ID,MANAGER_ID),
FOREIGN KEY (MANAGER_ID) REFERENCES EMPLOYEE(EMP_ID)
);
Here's the table it is referencing:
CREATE TABLE EMPLOYEE(
EMP_ID NUMBER(2) NOT NULL,
NAME VARCHAR(40),
OFFICE VARCHAR(20),
EXPERT_ID NUMBER(2),
PRIMARY KEY (EMP_ID,EXPERT_ID),
FOREIGN KEY (EXPERT_ID) REFERENCES EXPERTISE(EXPERT_ID)
);
Whenever I try to run the script it always comes back with:
Error report -
SQL Error: ORA-02270: no matching unique or primary key for this column-list
02270. 00000 - "no matching unique or primary key for this column-list"
*Cause: A REFERENCES clause in a CREATE/ALTER TABLE statement
gives a column-list for which there is no matching unique or primary
key constraint in the referenced table.
*Action: Find the correct column names using the ALL_CONS_COLUMNS
catalog view
I've looked around but can't seem to find the problem. Any help would be appreciated.
Here's the full code (haven't tested the last table yet):
CREATE TABLE EXPERTISE(
EXPERT_ID NUMBER(2) NOT NULL,
DESCRIPTION VARCHAR(50),
HOURLY_RATE NUMBER(3,2),
CHARGE_RATE NUMBER(3,2),
PRIMARY KEY(EXPERT_ID)
);
CREATE TABLE EMPLOYEE(
EMP_ID NUMBER(2) NOT NULL,
NAME VARCHAR(40),
OFFICE VARCHAR(20),
EXPERT_ID NUMBER(2),
PRIMARY KEY (EMP_ID,EXPERT_ID),
FOREIGN KEY (EXPERT_ID) REFERENCES EXPERTISE(EXPERT_ID)
);
CREATE TABLE ASSIGNMENT(
ASSIGN_ID NUMBER(2) NOT NULL,
START_DATE DATE,
END_DATE DATE,
BUDGET NUMBER (10,2),
MANAGER_ID NUMBER(2),
PRIMARY KEY (ASSIGN_ID,MANAGER_ID),
FOREIGN KEY (MANAGER_ID) REFERENCES EMPLOYEE(EMP_ID)
);
CREATE TABLE ALLOCATION(
EMP_ID NUMBER(3) NOT NULL,
ASSIGN_ID NUMBER(3) NOT NULL,
DAYS_WORKED_ON DATE,
HOURS_WORKED_ON DATE,
PRIMARY KEY(EMP_ID,ASSIGN_ID),
FOREIGN KEY(EMP_ID) REFERENCES EMPLOYEE(EMP_ID),
FOREIGN KEY(ASSIGN_ID) REFERENCES ASSIGNMENT(ASSIGN_ID)
);
I'm using Oracle SQL Developer to make it

Answer 1

*Cause: A REFERENCES clause in a CREATE/ALTER TABLE statement gives a column-list for which there is no matching unique or primary key
constraint in the referenced table.
The problem is that EMP_ID (by itself) isn't a primary or unique key of table Employees, instead, you have a compound primary key (EMP_ID, EXPERT_ID).
To fix the issue either make EMP_ID the primary key of the Employees table (which seems intuitive as each employee ought to have a unique id) or add a separate unique constraint on EMP_ID.
As pointed out in the comments, if you make EMP_ID the primary key, then (EMP_ID, EXPERT_ID) will also be unique by extension.

Answer 2

As the error suggest, the column you've referenced a foreign key doesn't match a unique constraint/pk on the parent table. Specifically for the primary key EMP_ID, EXPERT_ID you reference only EMP_ID.