If you are getting this error check the following:

1: Right click on References
2: Add the reference to the assembly that is missing (this might be another project that you have).
3: In using section (imports for VB.NET) add:
Using AssmblyName

Also, make sure that the Target Framework are same for both assmeblies (the one that is being called and the calling project).

Check Build Action is set to Compile.