"I am very happy that you are capable of learning a new language."
Translation:Mi estas tre feliĉa, ke vi kapablas lerni novan lingvon.
Ŝajnas ke "Mi tre ĝojas ke vi povas lerni novan lingvon." malbonas. Ĥmmmm, ĉu vere?…
I don't see any mistakes with your sentence. But Duo wants you to practice the verb kapabli, which is slightly different from povi. Capable means that a person has the ability to do something. "Can" means they not only have the ability, but conditions are so that they aren't prevented from doing it.