RDL build in visual studio 2015 not opening\supported in 2012. How to make compatible in old version.
Recent versions of SSDT have possibility to set different versions as targets (2005. 2008R2, "2016 and later" etc).
This option is available under Project Configuration properties->General->TargetServerVersion
Consider to update SSDT to latest version (17.4), However even older versions have a possibility to downgrade RDL by rebuilding project (right click->rebuild) and copy RDL from the output folder (bin\Debug) if target version was set to a previous release