Error CS0234: The type or namespace name ‘HelloWorld.Core’ does not exist in the namespace ‘HelloWorld’

Warning MSB3245: Could not resolve this reference. Could not locate the assembly “HelloWorld.Core”. Check to make sure the assembly exists on disk. If this reference is required by your code, you may get compilation errors. Error CS0234: The type or namespace name ‘Core’ does not exist in the namespace ‘Document360’ (are you missing an assembly … Read moreError CS0234: The type or namespace name ‘HelloWorld.Core’ does not exist in the namespace ‘HelloWorld’