TFS 2015 view content doesn't show any code

Question

We have a TFS 2015 server created, and none of the developer in our team will be able to see the file content. In Pull Request part, it is still the same thing which makes the code review really difficult. We can only see the code content if we click on the folder instead of the file, but it will list all file content inside this folder. I checked the setting part, but I couldn't find any part that could possible relates to this problem. Please advise, thanks.