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There are a few possible reasons why the filter option is disabled in Excel. Here are some common scenarios and solutions:
• You are using a shared workbook. Filtering is not available in shared workbooks, unless you unshare them first. To do this, go to Review > Share Workbook and uncheck the Allow changes by more than one user at the same time option. Click OK and save the workbook. Then, you can add and use filters as usual.
• You are using a protected worksheet. Filtering is not available in protected worksheets, unless you unprotect them first. To do this, go to Review > Unprotect Sheet and enter the password if prompted. Click OK and then you can add and use filters as usual.
• You are using an Excel table. Filtering is automatically applied to Excel tables, so you do not need to add filters manually. You can use the drop-down arrows in the table headers to filter the data. If you do not see the drop-down arrows, make sure the Table Tools Design tab is selected and check the Filter Button option in the Table Style Options group.
• You are using an older version of Excel. Filtering may not be compatible with some older versions of Excel, such as Excel 2003 or earlier. You may need to upgrade to a newer version of Excel to use the filter feature.