I shall is not a grammatical error, even if you prefer to use I will which once would have been incorrect.
Then report it as "my answer should be accepted." Griping about it in the forum won't help.
Duolingo does not like your using "shall" for the future tenses. It is really an emphatic "Will." Just use will and everything will be OK.
although IMO it would have been an error in say, 1950s England if it had been used when "I will" was required. :)
Two Oxford University professors of English were walking along the banks of the Isis one day, when they heard a student screaming urgently from the waters, "I will drown, no one shall save me". Thereby convinced of the student's determination to descend to a watery grave, they continued in their perambulation and left him to his fate.
Brilliant! thanks, Stevestewart0001. Although, even if he'd cried, "no one will save me", it wouldn't have done him much good unless someone called Nemo happened to be walking past.