redis.admin_display route not found

The issue here is , route needs to be defined in routing.yml file.

redis.admin_display:
  path: 'redis/demo/page'
  defaults:
    _form: '\Drupal\redis\Form\RedisDemoForm'
    _title: 'Redis configuration'
  requirements:
    _access: 'TRUE'

It was the .info.yml having a non-existing route in its configure key.

Don't know why I only see a error on one of my project now, as it as not been changed for many years.

Definitely shouldn't be an invalid route. Not sure if we should have that at all, there isn't really anything to configure there, but doesn't hurt I guess.