Need to do an IFERROR formula in a 3rd colunm.

Guindina 

You can use data validation to prevent users from entering values in columns C and D if column A has a value greater than 0.

Here’s how you can do this:

1. Select the cells in columns C and D where you want to apply the data validation.
2. Click on the Data tab in the ribbon and select Data Validation from the Data Tools group.
3. In the Data Validation dialog box that opens, select Custom from the Allow drop-down menu.
4. In the Formula field, enter the following formula: =IF($A1>0,FALSE,TRUE)
5. Click on OK to apply the data validation.

Now, if a user tries to enter a value in columns C or D when column A has a value greater than 0, they will see an error message and will not be able to enter the value.

You can also customize the error message that is displayed by clicking on the Error Alert tab in the Data Validation dialog box and entering your desired message.