It is very simply, your linker, from the looks of it MSVC/MSVCPP saying you are compiling stuff, but not providing the appropriate links from one file to another.
The linker says you promised the compiler you will supply information later, but you do not.
For example, you have a Header file included in your project, but never told it to also compile and link the C or C++ file that belongs to it.
Or one of several other options for the linker not finding what it needs. But the missing C/C++ file is the most common cause for the linker complaining about not finding stuff.
If this means nothing to you, best to google terms like "Add existing source files to MS Visual C++ project", if the instructions are at the least half baked, it'll include making sure it gets compiled and linked.
Up to you if you find this posting worthy of your donations.
Best regards,
Robert van Leeuwen
Embedded HW and SW engineer
Asmyldof (Chamber of Commerce Registered, The Netherlands)