undefined method `add_class' for #<Spree::Core::Engine::SpreeCalculators:0x000000065496c8>

[ebadmail]

Hi,

I tried to add the spree_product_customizations gem during the installation I have this error.

rake aborted!
NoMethodError: undefined method add_class' for #<Spree::Core::Engine::SpreeCalculators:0x000000065496c8> /home/canada-post-app/.rvm/gems/ruby-2.3.1@canada-post/bundler/gems/spree_product_customizations-feeae5ca27ba/lib/spree_product_customizations/engine.rb:23:in block in class:Engine'

I'm using
gem 'spree', '> 3.6.4'
gem 'rails', '> 5.2.1'

Do you know what is the issue?

Thanks,