Hello Sherry,
Thank you for posting question on Microsoft Windows Forum!
Based on the issue description. It is highly likely that the version mismatch is the primary cause of your compatibility issues. While both Excel 2021 and Excel 2024 use the same .xlsx file extension, the Excel 2024 calculation engine includes dozens of new functions and features that Excel 2021 simply does not recognize.
When a file moves from a newer version (2024) to an older one (2021), the software encounters instructions it has not been programmed to understand. Excel 2024 introduced over 25 new functions. If your source files use any of these, Excel 2021 will display a #NAME? For Dynamic Charts. Excel 2024 allows charts to reference dynamic arrays (ranges that change size automatically). In Excel 2021, these charts may appear static, fail to update, or lose their data references entirely. Also, Excel 2024 defaults to OpenDocument Format (ODF) 1.4, whereas Excel 2021 only supports up to ODF 1.3. If your files are saved in this format, you will see significant formatting and structural errors.
The suggestion here is to run the Compatibility Checker. On the PC with Excel 2024, go to File > Info > Check for Issues > Check Compatibility. This tool will list every specific cell and formula that will break in Excel 2021. On the other hand, If the users in 2021 only need to view the data, consider copying the 2024 data and using Paste Values to remove the problematic formulas before sharing.
Hope the above information is helpful! If it helps you getting more insight into the issue, It is appreciated to consider clicking "Accept Answer".