An _xlfn. prefix is displayed in front of a function in a formula. When the formula is calculated, it displays the #NAME? error value.

The _xlfn. prefix in front of a function in a formula indicates that the function is not recognized by Excel. This typically happens when the function is part of a newer version of Excel that is not available in the version you are using, or if the function is misspelled or incorrectly referenced. When Excel encounters an unrecognized function, it returns the #NAME? error value, indicating that it cannot interpret the function name.

To resolve this issue, ensure that you are using the correct function name and that your version of Excel supports it. If the function is from an add-in or a custom function, ensure that it is properly registered and accessible in your Excel environment.

References:

• Excel Worksheet and Expression Evaluation
• Calling into Excel from the DLL or XLL
• Troubleshoot custom functions
• Excel Worksheet and Expression Evaluation
• Excel Worksheet and Expression Evaluation
• Calling into Excel from the DLL or XLL
• Excel Worksheet and Expression Evaluation

That _xlfn. prefix in front of Excel formulas indicate the worksheet was created in a different version of Excel and your version of Excel does not support the formulae used in the spreadsheet, most likely, the worksheet was created in the Microsoft 365 version of Excel, what version of Office do you have installed on your PC?