Using TableField with Node:Create?
This has not much to do with form API, only with the structure of the field.

You need to pass in the values that the field type expects, based on its propertyDefinitions() method for example. Looking at http://cgit.drupalcode.org/tablefield/tree/src/Plugin/Field/FieldType/TablefieldItem.php?h=8.x-2.x#n141, it does some special processing on it, so might have to account for that.

The easiest option for that kind of stuff is usually to just create a node in the UI, then load it and call $node->toArray(). The structure that you get back ther for your field is usually also what it expects to be set.