Решено Переименование в frontend app/javascript в Rails 6

# In config/webpacker.yml, change the name of the folder as follow:
mv app/javascript app/frontend

# Inside webpacker.yml, first lines  
default: &default  
source_path: app/frontend # Change here  
source_entry_path: packs

$ bin/rails assets:clobber
$ bin/rails webpacker:compile