Start Excel in Safe Mode and check if the keyboard shortcut works then. If it does, one of the add-ins you have installed and activated is overriding the built-in shortcut.
To start Excel in Safe Mode, open the Start menu, find the Excel icon, hold down the Ctrl key and click the Excel icon. This will start Excel without any add-ins.
Check if the shortcut works now.
In that case,
- start Excel in normal mode
- disable all add-ins,
- then enable add-ins one by one
- Every time you enable an add-in, close Excel, open it again in normal mode
- then see if the keyboard shortcut works.
Once you have identified the add-in that takes over the keyboard shortcut, disable the add-in. If disabling is not an option, learn how to create an autosum by clicking the autosum icon on the ribbon instead of using the keyboard shortcut.
For more info about safe modes in Office, see here