Error: L6218E: Undefined symbol ADC_Cmd (referred from adc.o)

This error message indicates that the linker is unable to find the definition of the symbol "ADC_Cmd" which is referenced in the file "adc.o".

To resolve this error, you need to make sure that the function "ADC_Cmd" is defined somewhere in your code, and that it is being linked properly.

Here are some steps you can take to debug and fix this error:

1. Check that the function "ADC_Cmd" is defined in your code. Look for its implementation in your source files or header files.

2. Make sure that the file containing the implementation of "ADC_Cmd" is included in your build system, and that it is being compiled and linked properly.

3. Check that there are no typos or errors in the function name "ADC_Cmd". Make sure that it matches the name used in the code where it is being called.

4. Check that the library or module that provides the implementation of "ADC_Cmd" is included in your build system, and that it is being linked properly.

5. If you are using a third-party library that provides "ADC_Cmd", make sure that you have included the correct header files and libraries in your build system.

By following these steps, you should be able to resolve the linker error and successfully build your code.