Please read through
- What's the difference between declaring and defining in C and C++ - Cprogramming.com
- c++ - How do I use extern to share variables between source files? - Stack Overflow
- visual studio - How to declare a global variable in C++ - Stack Overflow
You’re missing
/* define global variables declared in header so that they actually exist */
struct Board1 Board_1;
struct Board2 Board_2;
struct Board3 Board_3;
struct Board4 Board_4;
struct Board5 Board_5;
struct Board6 Board_6;
struct Board7 Board_7;
struct Board8 Board_8;
struct Board1PV Board_1PV;
struct Board2PV Board_2PV;
struct Board3PV Board_3PV;
struct Board4PV Board_4PV;
struct Board5PV Board_5PV;
struct Board6PV Board_6PV;
struct Board7PV Board_7PV;
struct Board8PV Board_8PV;
struct Board1LOW Board_1LOW;
struct Board2LOW Board_2LOW;
struct Board3LOW Board_3LOW;
struct Board4LOW Board_4LOW;
struct Board5LOW Board_5LOW;
struct Board6LOW Board_6LOW;
struct Board7LOW Board_7LOW;
struct Board8LOW Board_8LOW;
struct Board1HIGH Board_1HIGH;
struct Board2HIGH Board_2HIGH;
struct Board2HIGH Board_2HIGH;
struct Board3HIGH Board_3HIGH;
struct Board4HIGH Board_4HIGH;
struct Board5HIGH Board_5HIGH;
struct Board6HIGH Board_6HIGH;
struct Board7HIGH Board_7HIGH;
struct Board8HIGH Board_8HIGH;
in one of your cpp files (after the include so the types are known)