<Import Project="..\packages\Microsoft.Bcl.Build.1.0.14\tools\Microsoft.Bcl.Build.targets" Condition="Exists('..\packages\Microsoft.Bcl.Build.1.0.14\tools\Microsoft.Bcl.Build.targets')" />
<Import Project="packages\Microsoft.Bcl.Build.1.0.14\tools\Microsoft.Bcl.Build.targets" Condition="Exists('packages\Microsoft.Bcl.Build.1.0.14\tools\Microsoft.Bcl.Build.targets')" />
(Note the change from ..\packages\ to packages\. It might be a different relative structure in your case, but you get the idea.)
@Simon_Weaver the position of your .csproj relative to your .sln does not matter in this context. What matters is whether anything that is referenced in your .csproj had moved somewhere else. If so, then you need to fix it. If you moved your '.csproj' with everything that it references intact, but kept your .sln where it was, then you'd have to fix the .sln file to the new location of .csproj-es, but there would be no need to edit .csproj files.
I had a similar issue, but really weird. I was using it in a subsolution module, so it was fine in that solution, but when I referenced that solution from another solution, the packages were in a different place. I changed ..\packages to $(SolutionDir)packages throughout the .csproj and that fixed it.
If you don't want to muck around with the .csproj file manually I've found that taking a note of all the nuget packages you have installed for the project, deleting them and reinstalling them resolved this issue for me. I was trying to remove a project from a solution to put into its own git repository when I ran into this issue.
In my case it happened after I moved my solution folder from one location to another, re-organized it a bit and in the process its relative folder structure changed.
Similar issue....I had moved the .csproj file up a level in the directory structure and had to change from "..\..\packages\..." to "..\packages\...".
does this mean your .csproj is at the same level as your .sln file?