There could be several reasons why you are unable to save new Excel spreadsheets.
Some possible reasons include issues with third-party add-ins, permissions, disk space, antivirus software, file sharing or file naming.
One way to troubleshoot this issue is to try saving the workbook using a new file name or moving the original worksheets to a new workbook.
You could also try saving the file as a different Excel file type or saving the workbook to another location.
If these steps do not work, you could try repairing your Office apps.
To do this, hold down the Windows key on your keyboard and press the R key.
In the box that appears, type “appwiz.cpl” and click “OK” or press the ENTER key.
Locate Office 365 in the list of apps and select it.
Click the “Change” button.
When the Repair Wizard appears, select “Quick Repair” and click the “Repair” button.
If this does not fix the problem, repeat these steps again choosing the “Online Repair” option.
I hope this helps!