This thread is resolved. Here is a description of the problem and solution.
Problem:
The issue here is that the user is unable to add a Post Form cell to their Layout.
Solution:
In this user's case it was being caused by the Woocommerce Custom product plugin and what I would recommend that you do is to temporarily disable the non-toolset plugin and try again.
This support ticket is created 6 years, 1 month ago. There's a good chance that you are reading advice that it now obsolete.
This is the technical support forum for Toolset - a suite of plugins for developing WordPress sites without writing PHP.
Everyone can read this forum, but only Toolset clients can post in it. Toolset support works 6 days per week, 19 hours per day.