Cannot Add Item To Cart

[mayordwells]

I am at the early of converting this repo into a marketplace application for this project www.macroshops.com.
Please help me I get this error when I try to add Item to Cart on localhost.
Every other things work as displayed in the heroku sample app. Except rails db:seed also did not work.
But the seed file is not as important as this error. Any idea how to fix this?

Warning: Failed propType: Invalid prop message of type object supplied to Flash, expected string. Check the render method of Flashes.

And on the Flash I get [object Object].
And in the console I get this Error.
ROLLBACK
23:32:03 rails.1 | [active_model_serializers] Rendered ActiveModel::Serializer::Null with Hash (0.11ms)
23:32:03 rails.1 | Completed 422 Unprocessable Entity in 655ms (Views: 0.8ms | ActiveRecord: 55.7ms)