This is easiest to reproduce if a subtle misconfiguration is created.
Create a redis instance with name, say "foo":

redis:
enabled: true
templates:
foo:
replicas: 1

and in the designate CR, set the RedisServiceName to {{ foo-redis }} :

designate:
enabled: true
template:
redisServiceName: "foo-redis"

This results in a backend_url configuration of

[coordination]
backend_url=redis://None:6379/?ssl=true

which is invalid.

To get the correct value, the redisServiceName would have to be changed to "foo" or the redis configuration changed to "foo-redis".

The situation where this was reported, the redis instance was named "designate" and the designate CR was left as a "designate-redis"

To work around this issue, either change the name of the redis instance to match the name in the designate CR. ie. foo-redis or change the name in the CR to match the name of the redis instance , i.e. "foo"