I was able to add custom fields in checkout page using WooCommerce Checkout Field Editor Pro plugin. But after adding around 15 fields I can't add more fields in the same section, it is saying "Your changes were not saved due to an error (or you made none!)." 


Seems it is an issue with the run-time configuration property max_input_vars value. Please try again after increasing its value. Below article may help you to increase max_input_vars value.

How to increase Max Input Vars?

In case you are not allowed to change the max_input_vars value, split the fields into separate sections instead of creating all fields in a single section. 

You can display newly created sections at below positions,

  • Before customer details  
  • After customer details  
  • Before billing form  
  • After billing form  
  • Before shipping form  
  • After shipping form  
  • Before registration form  
  • After registration form  
  • Before order_notes  
  • After order notes