How come clients can't fill in a table?

If you find that clients aren't able to fill in a table, you should first check to see if the worksheet is a form-based one.

A Form-based Worksheets contain at least one form item (such as a text area, dropdown menu, checkbox, and so on). In Form-based Worksheets, input is restricted to the form items present in that Worksheet.

In other words, clients can't freely write wherever. Since a table isn't considered a form item, clients can not fill out tables in a Form-Based Worksheet.

However, clients can type directly into the cells of a table in a WYSIWYG-based (i.e. non-form-based) Worksheet.

If you prefer to use a form-based Worksheet and want your clients to fill out items of a table, you'll simply need to add inputs to each cell that your client needs to be able to fill out.  Usually simple text input will do, but you may choose a multi-line text area as well.