Error message on attempted verification of Marlin firmware

The extruder thermistor circuit seems to have died on my Wilson II printer so I would like to move the thermistor contact from its present position on the board to one of the two redundant thermistor slots. I have changed the pin numbering in pin.h but when I verify the Marlin file, I get an error message at line 139 in the SD2Card.h file. This line reads as follows:

#define SD_CHIP_SELECT_PIN SS_PIN

and the error message is:

'SS_PIN was not declared in this scope'.

The program seems to be expecting an entry of some type. What does this mean and how do I correct it. Your help would be truly appreciated. I get the same message with 1.6.10 and 1.8.12 so it does not seem to be an IDE issue.

It means you don't have a definition for the Slave Select pin.

Maybe the Marlin forum knows more?

Thanks for the prompt response; I will check with the Marlin forum.