[Forminator Pro] Help with custom validation

We’re looking forward to implementing some validation in a Forminator field following the instructions on this post – https://premium.wpmudev.org/forums/topic/forminator-pro-validation-letters-and-numbers-only/ , but unfortunately, we can’t make it work. We need a different validation though, we need to allow only a-z A-Z.

The mu-plugin is currently added to the site. Could you please take a look and see why it isn’t working?