Why does this not allow/follow the subject-time-verb form: 我这次买单？
My Chinese teacher has taught that the use of 次 is flexible; 我这次买单 should be accepted.
A native Chinese clarified this for me. She said that the normal style is 这次我买单, but it is also acceptable to say it in the other order 我这次买单.
Perhaps Duolingo will get around to adjusting this eventually.
is 我这次付钱 not correct?
"maidan" should be written as "埋单“. "买“ is a very common spelling error made by those who are ignorant of the word's Cantonese orgin.
I'm pretty sure that's absolutely incorrect.
The structure of the sentence is correct.