It sounds like the cell has data validation applied to it, which restricts the values that can be entered. Here's how you can check and modify the data validation settings:

1. Select the cell that is giving you the error message.
2. Go to the "Data" tab in the ribbon at the top of the screen.
3. Click on "Data Validation" in the "Data Tools" group.
4. In the "Settings" tab of the "Data Validation" dialog box, you should see the criteria that are restricting the values that can be entered into the cell.
5. If you want to modify the criteria, make the changes in the "Settings" tab and click "OK".
6. If you want to remove the data validation entirely, select "Clear All" in the "Data Validation" dialog box.

Once you've made any necessary changes, you should be able to enter values into the cell without receiving the error message.