Kubernetes Let's Encrypt cert-manager Error secret not found

Error preparing issuer for certificate coffeer-ci/mydomain.fr: http-01 self check failed for domain "mydomain.fr"

Means that it can't do the HTTP check that you actually own the domain. Do you own mydomain.fr? If yes, you need to add a DNS entry to make mydomain.fr resolve to the external IP (A record) of the load balancer (or if the load balancer has a name entry it would have to be a CNAME record, in the case of AWS ELBs) This way letsencrypt can use it to verify that you own the domain.

The other error:

Warning Sync 3m (x49 over 1h) loadbalancer-controller Error during sync: googleapi: Error 403: Quota 'BACKEND_SERVICES' exceeded. Limit: 9.0 globally., quotaExceeded

looks like the byproduct of not being able to verify the domain. If you don't specify and Ingress looks like cert-manager creates one for you with a 'LoadBalancer' type of service. It looks like it creates it initially but it keeps trying to sync to create it on GCP (maybe because to check if it can configure port 443) but after a while, the GCP API is throttling you.