I created some reports in Visual Studio 2015 with all the latest updates. However, when I try to deploy the reports I get this message:
The definition
I ran in to same problem and this is how I solved it,
Your source reports format and namespace will be updated to latest version. But bin folder reports will be build to be compatible with targeted reporting server version.
If you are having trouble in a Visual Studo 2017 C# desktop application with LocalReport (RDLC), please see this answer:
https://stackoverflow.com/a/45149488/6732525