块 should be accepted to any kind of currency, not just Yuan. It technically means "piece" and it's usually used to refer to money.
Isn't 〇 usually used for 零, or did I ｍｉｓｈｅａｒ ｍｙ ｔｅａｃｈｅｒ. Ｉ ｄｏｎ'ｔ ｋｎｏｗ ｗｈｙ ｍｙ Ｅｎｇｌｉｓｈ ｔｅｘｔ ｓｕｄｄｅｎｌｙ ｌｏｏｋｓ ｌｉｋｅ ｔｈｉｓ.
A rule of thumb: If the number is just read out, like room numbers, phone numbers, years, etc., both 零 and 〇 are available. If the number represents a real value, like prices, measurements, then you'd better just use 零. This means that 零 is the safest word.
Chinese input has "whole" 全形 or "half" 半形, it can be changed by using spacebar + shift.
全形 Ａ Ｂ Ｃ Ｄ 半形ABCD
全形letters can be seen but may not be understood as letters by machines.
If zero is necessary for the missing hundreds, is it not necessary for the units too?
It would be more useful if the numbers used at this point in the exercise were different from those used earlier in the exercise, and different from those used in the Tips. Otherwise, it just becomes a question of memory rather than understanding and applying the grammar point. This applies to most Duolingo exercises, which are too repetitive.
This is wrong. 六千零七十块 = 6070 KUAI, which is an accepted and very common English translation (at least on the mainland). 6070 YUAN should be 六千零七十元.