Can't add new field

Hi – I just downloaded the plugin. I am unable to add any new fields. Every time I click on the “Add new field” button in each of my contact forms, I get an error message page that says “Forbidden,” with a Back arrow to return to the contact page. Can you add not new fields in the free version anymore? I’ve used this plugin on other sites with no issues until now. Thank you!

https://www.ads-software.com/plugins/si-contact-form/