Here are some techniques to debug errors in complex formulas with multiple functions in Excel:

1. Isolate the Error:

* Check for typos: Ensure all function names, cell references, and operators are spelled correctly.
* Evaluate the Formula Step-by-Step: Use the "Formula Evaluator" (Alt + T + E) to break down the formula step by step. This allows you to see the intermediate results of each function within the formula.
* Test Sub-sections: Break down the complex formula into smaller parts and test each section individually. This helps pinpoint where the issue might be originating.

2. Verify Function Arguments:

* Function Reference: Double-check you're using the correct function for the desired task. Refer to Excel's documentation for specific function arguments and their expected data types.
* Cell References: Make sure cell references are accurate and point to the intended data. Use absolute references ($A$1) if necessary to avoid unintended changes when copying formulas.
* Data Types: Ensure the data types used by the functions are compatible. For example, a function expecting numbers might not work correctly with text values.

3. Break Down Complex Logic:

* Nesting: If your formula has many nested functions, consider simplifying it by using helper columns with intermediate calculations. This can make the logic easier to follow and debug.
* Logical Operators (AND, OR): Double-check the logic of your AND/OR operators. Ensure they are placed correctly to achieve the desired outcome.

4. Error Handling Functions:

* Use `IFERROR` or `ISNA`: Consider using error handling functions like `IFERROR` or `ISNA` to gracefully handle potential errors and return a custom message or value if an error occurs. This helps prevent formula breakage and aids in debugging.

5. Leverage Excel Features:

* F9 Key: Use the F9 key to evaluate individual parts of the formula and see their results. This helps isolate problematic sections.
* Trace Precedents/Dependents: Use the "Trace Precedents" and "Trace Dependents" features to track which cells feed data into your formula (precedents) and which cells are affected by your formula (dependents). This can help identify circular references or unintended data flow.

6. Online Resources and Community Support:

* Excel Help: Consult the built-in Excel help documentation for detailed information on specific functions and troubleshooting tips.
* Online Forums: Search online forums and communities dedicated to Excel for solutions to similar errors you might be encountering.

Remember: Debugging complex formulas can be a process of elimination. By systematically checking each element and utilizing the tools available in Excel, you can isolate the source of the error and get your formula working correctly.