I agree. It technically makes sense to say 二百, but no one says that.

You are correct. Duolingo is wrong.

二百 • Èrbǎi • 200 • Two-hundred

[ 二百 ] ≠ [ 二 百 ]

Èrbǎi ≠ [ Èr Bǎi ]

Èrshí • 二十 • 20 • Twenty

Written Chinese Numbers

0-20: Zero, One, Two, ..., Ten, Eleven, Twelve, ..., Twenty • [ In the English language, 21 unique words for the names for integer numbers 0-20 ]

It seems "wording" or Chinese Cardinal character number notation treats the name for a Cardinal number as single word, [ even if the 0-999 value concatenate characters ] not unlike in English: zero, ..., nine, ten, twenty, thirty, ..., ninety in English wording. Regardless of the mathematical notation numbering system radix base number in use, Unary, Binary, Roman, Quaternary, Octal, Denary, Hexadecimal, Sexagesimal, and etc.,

A • Ten • Hexadecimal
XIX • Nineteen • Roman
13 • Nineteen • Hexadecimal
19 • Nineteen • Denary
23 • Nineteen • Octal
43 • Nineteen • Quaternary
10011 • Nineteen • Binary
1111111111111111111 • Nineteen • Unary

And what is the pronunciation ?

But for taking kanji, it could help to take Chinese, since technically kanji is Chinese characters.