"Early in April"
Yes, but that is the way to express this in German, you will rarely hear a German say früh im [Monatsname].
To me 'Anfang' means 'Start' or 'beginning'. It may be that Germans rarely say 'fruh im' in this situation. Could that be because they do not mean 'early' but 'at the beginning'? Is this another instance of Duo's rather sloppy use of English?
Would "In the beginning of April" or "At the start of April" not be the same as "early in April" here? If so, why not? Duo doesn't accept it but I don't understand the difference, especially given some definitions of "Anfang" as "beginning" or "start".