blacklist_migration.rb

class CreateJwtBlacklist < ActiveRecord::Migration[5.0]
  def change
    create_table :jwt_blacklist do |t|
      t.string :jti, null: false
    end
    add_index :jwt_blacklist, :jti
  end
end

there's a typo on line 6, it should be plural when adding index (same as with table name):
add_index :jwt_blacklists, :jti