If you want to set all items to have a Most Recent Cost of zero, the easiest way of doing this is to:

1a) Use Export Items to export all your item codes to an Excel spreadsheet. You only need to export the Item Number and Most Recent Cost fields and no others.
or
1b) Use the SmartFinder on the Item code to export all your item codes to Excel Spreadsheet. You only need to export the Item Number and Most Recent Cost fields and no others - use the Column Editor to define the columns. One advantage here is that you can list only the items that have a Most Recent Cost not equal to zero before exporting, so you know where the culprits are.

2) Edit the Excel spreadsheet and ensure the Most Recent Cost column only contains zero amounts.

3a) Use the Import Items function to import the item changes. In the import template, make sure that the Add Items checkbox is not selected (you are not adding new items) and also the Overwrite Items checkbox is selected (you want to make changes to your existing items). For the selected fields, you only need the Item Number and Most Recent Cost fields and no others.
or
3b) Use the same procedure as above but only select the Item number field to import, and a Filler for the second field - to skip the Most Recent Cost in the Excel file. Then set a Default Field on the third tab to set Most Recent Cost to 0.00. This will then import 0.00 for Most Recent Cost for all items.

Make a backup of your data before attempting this, or other processes that make significant changes to your data.