I am also getting similar error


DQKern_ProcessHT.Chap11.Two.Main.c: In function 'DQKern_ProcessHT_Chap11_Two_Main_eqFunction_64':
DQKern_ProcessHT.Chap11.Two.Main.c:299: error: '$PG$lB11$rB' undeclared (first use in this function)
DQKern_ProcessHT.Chap11.Two.Main.c:299: error: (Each undeclared identifier is reported only once
DQKern_ProcessHT.Chap11.Two.Main.c:299: error: for each function it appears in.)
\MinGW\bin\mingw32-make: *** [DQKern_ProcessHT.Chap11.Two.Main.o] Error 1
\MinGW\bin\mingw32-make: *** Waiting for unfinished jobs....
Compilation process failed. Exited with code 2.

This error ended up being caused by an index that was out of range for certain conditions. The IB2 was the second element of that array, and there was a case where that could have been out of the size of the array. I ended up fixing it with an if-then statement to make sure the equation was valid for all potential sizes of the array.