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Could you please share with us what version of Office are you using now?
You could go to File > Account > Product Information to get the version name.
Generally, Microsoft Excel will claim such information when saving a worksheet as any format other than .xls or .xlsx.
Under you case, the Text (MS-DOS) format (.*txt) does not support some Excel features, such as formulas, (conditional) formatting, charts and ect.
So you would receive this message by design.
If you want to only save data, I suggesy you ignore this message, and click "Yes" like the following image shown to save it as a Text (MS-DOS) file.
Besides, if you do not want to see this prompt, it's recommended to try to add a registry subkey.
HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Excel\Options
Value name: DisableSaveAsLossWarningOpenDocumentSpreadsheet
Value type: DWORD
Value data: 1
0 - Display the warning message
1 - Suppressthe warning message
(You need to be cautious when modifying the registry. Improper modification may cause some problems.
It is recommended to back up registry keys before modification, just in case.)
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