Did you add the header file to include fstorage? An undefined reference means that it is unable to compile the command as you have not defined fstorage in your program. You can refer this thread to know how to solve this issue. In your case, look at the example flash_fstorage.
Even if you have added the header files, any relevant paths and related files should also be included and the module should be enabled in the sdk_config.h. Kindly go through the link I shared.
Also, check if you have you have added the .c file into SRC_FILES in the Makefile? This could be the problem if you have already added all the relevant files, but still have an error.
