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This error message typically occurs during the linking phase of compiling a program. It means that the compiler was unable to find a definition for a function or variable that was declared in the code.

Possible causes include:

- A typo in the function or variable name
- Missing include files or libraries
- The function or variable was not defined in the source code
- The function or variable was defined in a different file, but that file was not included in the compilation process
- The function or variable was defined in a library that was not linked to the program

To fix this error, you need to ensure that the function or variable is defined and that it is accessible to the compiler. This may involve checking your code for typos or missing include files, or adding the necessary library or source files to the compilation process.