data validation

one mark for a list-type validation, one for exactly the two items — Data Validation's "List" option is specifically what restricts entry to a fixed set of choices, as opposed to other Allow types (Whole Number, Decimal, Date) that restrict by kind of value rather than by an exact approved list. If Excel says the source is wrong, the separator setting is why, not the spelling: the character that separates list items is read from the computer's regional settings (comma in many locales, semicolon under South African settings, since the comma is already the decimal separator there), so a perfectly spelled list with the wrong separator for that machine's region is rejected as malformed, not as a spelling mistake. If prompted, tick "Apply these changes to all other cells with the same settings" so the fix applies consistently across the whole column rather than just the one cell you edited.