As explained in the capistrano-rails README, if you don't use the asset pipeline, then exclude capistrano/rails/assets from your Capfile. In other words:
# Instead of this:
require "capistrano/rails"
# Require only these:
require "capistrano/bundler"
require "capistrano/rails/migrations"
However, if you are using Webpacker, then there is a good chance that you still need to run asset-pipeline tasks. Notably, you still probably need to run assets:precompile during deployment, in order to trigger the webpack build.
In this case, you would need a more precise exclusion of the backup_manifest task. Following the instructions in the official Capistrano docs, you can clear the default implementation as follows:
# In deploy.rb
Rake::Task["deploy:assets:backup_manifest"].clear_actions