A FormWork Excel form is associated to a set of validation rules that Excel can evaluate so to avoid the use of unauthorized data.
When you press Write button, the Excel Client export your sheet data and evaluate them. If the validation fails, you will receive an error window like this:
It is an Excel dialog that inform the user data has not been exported. In this case the validation error is related to the column "Category Id", that cannot accept empty values.
TIP: spaces in column names are indicated with the text _x0020_ so in the exemple above "Category Id" is shown as "Category_x0020_Id".