The user needs to tell the compiler how much space should be reserved in RAM for various functions. In this case, the 'Interrupt RAM' space has 39 bytes reserved for it in bank 0. The compiler has detected that more RAM space is required. To reserve more space, go to the Project menu in PSoC Designer and select Settings > Linker, and change the value of "Relocatable code start address" to a larger value. In this case, the setting was 150. Increasing it to 160 solved the problem. Set it only as high as you need it, or you will waste space in RAM.