insmod: ERROR: could not insert module pin4_driver2.ko: Unknown symbol in module

This error message indicates that the module pin4_driver2.ko could not be inserted into the kernel because it contains an unknown symbol. This means that the module is trying to use a function or variable that is not defined or exported by the kernel or any of the loaded modules.

To fix this error, you need to identify the unknown symbol and make sure that it is available to the module. This may require modifying the kernel or the module code, or loading additional modules that provide the missing symbol.

You can use the 'dmesg' command to view the kernel logs and look for any additional information about the error. This may provide clues about the missing symbol and how to resolve the issue.