OK, lets look at the important part of your function

Begin 
    user := SYS_CONTEXT('userenv', 'SESSION_USER'); 
    select maPhong into v_maPhong from nhanvien where maNv = user; 
    return 'maPhong = ' || v_maPhong; 
End; 

The string that comes back will be added as a WHERE clause to your query. Let's say that v_maPhong comes back as CONNOR.

So a query such as:

select * from NHANVIEN

will now read like:

select * from NHANVIEN
where maPhong = CONNOR

That is wrong, because there is no column called CONNOR. You need to take care of the quotes as well. So a function like:

Begin 
    user := SYS_CONTEXT('userenv', 'SESSION_USER'); 
    select maPhong into v_maPhong from nhanvien where maNv = user; 
    return 'maPhong = ''' || v_maPhong || ''''; 
End; 

would return

maPhong = 'CONNOR'

which should then work in your query.