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 Worksheet contains at least one form item (such as a text area, dropdown menu, checkbox, and so on), which restricts inputs 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 won't be able to fill out tables within the worksheet until you add inputs into the table where the client needs to input information:

 Usually, simple text input will do, but you may choose a multi-line text area as well.

Keep in mind, clients will be able to type directly into the cells of a table in a WYSIWYG-based (i.e., non-form-based) Worksheet.