Creating a constraint
You'd probably be better off making this a bit field with NOT NULL or otherwise you also need to validate that only the strings 'Y' and 'N' were entered.

With the bit column arrangement the check constraint would be

CHECK (1 IN (Completion, Loading))


Otherwise with char(1) the check constraint could be

CHECK ('Y' IN (Completion, Loading))


but you'd need two additional column level constraints ensuring Completion IN ('N','Y') and similar for Loading.