Currently SimpleForm does not support ActionText's rich_text_area tag.
There is a pending open PR for adding ActionText's rich_text_area to SimpleForm.
An alternative way, while we wait until this above PR is merged, is to create a SimpleForm custom input within your application
To create an ActionText custom input for rich_text_area in SimpleForm simply add this code to your application below:
# put this in app/inputs/rich_text_area_input.rb