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There could be several reasons why this is happening:

1. The variable may not be defined or declared properly.
2. The scope of the variable may not be compatible with the function where it is being used.
3. The variable may not be passed correctly from one function to another.
4. There could be a naming conflict with another variable or function.
5. There could be a syntax error in the code that is causing the problem.
6. The variable may be of a different data type than expected.

To fix the issue, it is important to check the code thoroughly and analyze where and how the variable is being used. Debugging tools can be helpful in identifying the exact issue.