Also, DuoLingo has lost its honor in general with its AI obsession and heavy layoffs; only a petaQ would use such a coward’s website.
The only way a true warrior can learn Klingon is the old ways - the Okrand books and tapes!
EDIT: Klingon Wiki is also helpful, as is KlingonSKA for searching words and Hol ‘ampaS for font-related stuff and digital versions of out-of-print Okrand tapes.