"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.)
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.