Hi Salvdangelo,
Just to replicate the issue I have created custom schema for Theme and also one form and navigation hierarchy for this custom schema.
Schema Definition:

Form :

Navigation Hierarchy :

Folder created by using above navigation hierarchy and also after creating folder ,click on advance properties and check for template name for me it's 'kkktheme'. This name have to be same in your schema and form screenshot shared above.

Using form to enter values , result is below. You can see Folder ID is coming automatically and no error for related to folder.

Point to check
1. Make sure you have set default value at schema level and use correct name as the name coming after creating folder using navigation hierarchy
2. In Form use correct model name (case sensitive)
3. In navigation hierarchy use correct schema name
Thanks,
Kapil Kochar