Your error is in this line <%= link_to "Delete Image", asset, method: :delete %>
You want to pass the customer as well
<%= link_to "Delete Image", [current_customer, asset], method: :delete %></li>
To be more clear you might want
<%= link_to "Delete Image", path_for(current_customer, asset), method: :delete %></li>
Also, you are not actually destroying the @asset in your destroy method :)
def destroy
if asset = current_customer.assets.find_by(id: params[:id])
asset.destroy! #or destroy without ! but then it could rollback in not destroy
end
redirect_to root_url
end