That json error relates to incomplete data being passed between the database and the layout form.
If you are familiar with your browser developer tools you can go to the site mentioned, http://datatables.net/tn/1
and find instructions on how to exactly diagnose the error.
However, I’m not a dev so I can’t tell you specifically what the error means. But I do see a couple items in your screenshots that could produce the error.
In the ‘Layout Properties’ image I see your form has no ‘Layout ID’ or ‘Title’. The Layout ID is the form’s identifier in the database so is required data, and the Title is also required. So with those two items missing I’m not surprised you got an error.
“Layout based visit forms”, or “layout based forms” AKA LBFs, which you are trying to create, are a very capable way of making custom data entry forms. But they are fairly complex and very particular about how they are built. A tutorial can be found in the OpenEMR documentation WIKI that explains step by step how to make a sample LBF.
The tutorial was written for an older version of OpenEMR but the steps should still apply.
Hope this helps!