One thing you could try is to check if the “Enable AutoComplete for cell values” option is turned on. To do this, go to the File tab in Excel and click on Options.
In the Excel Options dialog box, go to the Advanced tab and scroll down to the Editing options section. Make sure that the “Enable AutoComplete for cell values” option is checked.
If this option is already checked and you’re still having trouble with autocomplete, you could try repairing your Office installation.
To do this, go to the Control Panel and click on Programs and Features.
Find your Office 365 installation in the list of programs and click on Change.
In the dialog box that appears, select Quick Repair and follow the prompts to repair your Office installation.
I hope this helps!