Where is the 要 in the English sentence?
要 in this context is used as a tense modifier. 要买 means "are going to buy". The correct modifier for "are buying" would be 在买. And if you wanted "have bought", the closest would be 买了.
I think it is implied in the word "are". The English can be interpreted two ways: that they are currently at the cash register buying, or that they intend to buy. The 要 would be in regards to the latter interpretation.
要 has many meanings (the "pop up" lists given by Duo are just the basic/ common meanings) 要 can also mean "going to" ("are buying" here).
一共 cannot go after 要买 in this case?
Same comment. It feels more correct after because it seems to be pointing to the people rather than the dresses when it's before.
Because the object of this sentence is 'dresses' I sort-of figured 'they' might be female. Sadly, 她们 is not accepted.
How did you not understand that they are drag queens?
Seems to be fixed now.
'are buying' should be 在买
The word "altogether" is vague. Is it referring to eight dresses in total or the fact that they, as a group, are buying the dresses?
一共 = "In total"
Once again, there are half a dozen ways to arrange this sentence in Chinese, but Duo only recognizes one.
要 is not necessary
they accept it without 要 now
now they don't again sadly
Reported about extra word (要) on November 2018. Will see if something is going to change.
they accept it without 要 now (0ctober 2019)
他们一共要买八条连衣裙. (tā men yī gòng yāo mǎi bā tiáo lián yī qún.) = Accepted.: 13 feb. 2020.
In Taiwan, "裙子" is "skirt," and "dress" would be "洋装" (traditional: 洋裝) yángzhuāng literally "foreign attire."
I think that word means both "dress" and "skirt" in Malaysia. I might be wrong though