I expected it to be "do not", which has a different connotation than "cannot". Are they interchangeable in Czech, and if not, why is it rendered as "cannot"?
This question repeats itself over and over. Are you American or British? Which exact connotation do you see and do you have some authoritative resources for that?
Here other native speskers say they would almost always use "can't"
Something like "Nemůžu vidět." would indeed sound very strange in Czech.
"I can’t see the donkeys" is accepted. If you have similar problems file a bug and report to Duolingo, we cannot help you, but first make sure did not have a typo in your answer. Always read your answer twice to check it is really correct.