When I am trying to call a method of another project class file, I am getting the following error

I have 2 C# library projects inside same solution file.
Inside each project I have a class file.
I have added reference of one project inside other.
There are NO compile time or build errors. During runtime, it is throwing the following exception (Image attached above)
My question is, how does one able to call a class method of one project from another method ?
Thanks in advance :)
Is your current project class file exist in the actual directory? The physical file might not be on the disk itself so it has this error message.
Simply put if your physical file exist in your directory, and if you wanna call that method in, be sure to put the namespace at the top and import that reference from your solution explorer.
Also, Do remember to include your method class as public.
Example Below: