Hi, thanks for sharing this issue.
Conditional project references can be problematic because they can cause misalignment between the project.assets.json file (generated at load time by VS) and the actual build step (executed by NCrunch in the workspace).
The easiest way to confirm whether the conditional reference is the source of the problem is to try commenting out the package reference option, and remove the condition from the project reference so that it's just a project reference. I'll be interested to see if this allows NCrunch to build the project. If it does, we need to make sure that NCrunch is set to use the same build configuration used by VS when it first loads the project and creates the project.assets.json file (via the Nuget restore step).
NCrunch can work fine with relative project references, and it can work with Nuget package references ... but it can't work with a project reference that was represented as a package reference during VS's Nuget restore step (or vice versa).
Edit: I've just checked the path given by the error and found it to be 271 characters long. NCrunch does support long file paths but it is reliant on everything in the toolset to be able to handle it, so in practice support for it is hit-and-miss. Moving your
workspace base path closer to the root of your disk might solve the problem.