Your datatable variable ‘in_FilteredDT’ has not been initialized. Doublecheck in your workflow that you either have a ‘Build data table’ activity saving to your datatable variable, or have some other activity that is outputting to the ‘in_FilteredDT’ variable
EDIT: It looks like in_FilteredDT is an argument. Make sure that you are passing in the value in your argument correctly when invoking the workflow. Also, make sure you don’t have a variable with the same name, as variables take precedence over arguments. This means you could pass in a value to the argument when making your workflow, but you also have a variable with the same name that does not have a value and is null. When this happens the variable is null and you’d get the error shown in the screenshot.