It's a beta course, so it's possible that there are (or were) mistakes.
But I think when we've seen would like, Duolingo has consistently used it with the subjunctive mood.
Emptor gemmas emere velit.
The customer would like to buy the gems.
Here's a tip with the verb conjugated in the present tense, subjunctive mood:
Here's the full conjugation:
"Gems" and "the gems" are both included as acceptable answers (and have been for several months). If your answer was marked wrong, please double-check that you didn't make another error/typo. (For instance, quite a lot of people are writing costumer instead of customer).