"Gallet ti olchi'r llestri."
Translation:You would be able to wash the dishes.
Gallet ti... is a conditional - 'you would be able...' or 'you could...'. You could also use Baset ti'n gallu/medru..., but with gallu/medru and hoffi/licio the short-form conditional verbs forms are quite often used in the colloquial language.
Is it possible to know why the answer cannot be "You could wash the dishes"
That is one of the alternative answers in the course.
If Duo is rejecting it, check that you have the latest version of the app if you use it. Otherwise, please raise a 'Duolingo bug report' so that Duo staff can investigate.