You need a / at the end of the procedure to terminate the statement and before the PL/SQL anonymous block:

CREATE OR REPLACE PROCEDURE evenorodd(X IN NUMBER) IS
BEGIN

IF MOD(X,2) = 0 THEN
DBMS_OUTPUT.PUT_LINE('even');
ELSE
DBMS_OUTPUT.PUT_LINE('odd');
END IF;
END;
/

BEGIN
evenorodd(15);
END;
/

db<>fiddle here