"Today I have neck pain."
Translation:Bugün bende boyun ağrısı var.
When you have a compound of the "X Y-(s)i" type and that compound is possessed, the possessive suffix replaces the -(s)i compound ending.
For example, sırt çantası wold be a backpack ("back bag"), but "my backpack" would be sırt çantam and "your backpack" would be sırt çantan.
And "his backpack" would be sırt çantası -- just like the basic compound. You can't tell the difference in that case. (But you could use onun sırt çantası to make it clear that the -sı is a "real" possessive in that case.)
"var" means "exist/present". So, there is no emphasis on who has the pain without "bende"
So far, you have learned, that "var" can be translated as "have". This is valid for most of the cases but it is not the direct translation of "have."
When you say; "(Benim) Üç elmam var.", you are actually saying that "My three apples are present/exist.". You are stating your belongings with possessive suffix.
For this example, "boyun ağrısı" is a compound name, and compound names tend to stick together. Therefore, either you should use "bende" to emphasise who has the pain, or you should break the compound name and add possessive suffix. The following two sentences have the same meaning and are accepted by Duolingo.
"Bugün bende boyun ağrısı var."
"Bugün boyun ağrım var."
most questions do not accept true answers.
I think you mean "correct answers" rather than "true answers".
If a given sentence is not accepted, please report it using the "my translation should be accepted" option.
Or at least say explicitly what should be accepted here. Just "fix these answers" is not helpful.