Visual Studio 2015 is still the supported IDE for UE 4.15. It sounds like you will need to work through the list of compile errors and address them individually (it’s possible there is just one root cause that triggers others). I recommend reviewing the Upgrade Notes section of the Release Notes for any changes to be aware of.
Edit for clarity: VS2017 is also supported as of UE 4.14, but there is no requirement to update
Cheers