I was recently upgrading some templates to VS2017 that I have in a VSIX package. Although most of my VSIX packages upgraded seamlessly I had one project that returned the following error:
The VSIX manifest must contain element 'Prerequisites'
None of my "vsixmanifest" files included a prerequisites section so I found it odd that one project complained while the others didn't. I digress though. The fix was to specify a prerequisite. According to MSDN, the CoreEditor is the default prereq that should almost always be included. So, I include that and voila, the VSIX compiles and works as expected.
Here is the XML section from my vsixmanifest:
<Prerequisite Id="Microsoft.VisualStudio.Component.CoreEditor" Version="[14.0,16.0]" />