That seems like the template you are using to publish such page is not using the DD4T Template Building Blocks or the format it uses to generate the output is incorrect. Double check the TBBs within the page template for such page, chances are you are publishing JSON and DD4T MVC app is expecting XML or you are not even publishing JSON/XML (due to the ...
Since, you are using DXA, I will say the approach of generating the sitemap might not makes sense (unless there are other fine prints which we are not aware about).
The correct approach could be similar to the one mentioned here:
Generate Sitemap XML File using default functions of DXA 1.5/1.7 in Web 8/8.5