how can I solve Secret tls-cert.default unknown issue?
You should try installing the Cert-manager with the ambassador. Edge stack supports the Let's encrypt in acmeProvider but only with one challenge HTTP-01.
Your HTTP-01 cert-challenge is getting failed while cert-manager supports the DNS-01 method also and if you want to use the wild card cert you can also use it.
https://www.getambassador.io/docs/edge-stack/latest/howtos/cert-manager/