This answer is not English!
Duolingo question: Write this in English?: Ich musste Englisch
My answer: I must learn English.
Duolingo answer: You used the wrong word. I'd to learn English.
Has Duolingo content not been reviewed properly? Unfortunately, their answer is meaningless.
There is a bug with auto-contractions on Duo, "is" get's contracted to apostrophe-s and "had" to apostrophe-d, even if it doesn't sound natual. This is what happened in your case.
"I had to learn English" is the correct translation of "Ich musste English lernen", like Kartoffeleis said.
I had to learn English not I must learn English