Ah, now we know your client.
And your environment.
But checking Octopus, there is an integrated Letsencrypt solution.
Octopus 3.16 or newer can integrate with Let's Encrypt to setup and manage the SSL certificate for the Octopus Portal. When the certificate nears its expiration date, Octopus will automatically renew the certificate with no intervention required.
That's always the best solution.
Looks like you have used an older configuration with an additional client.
And the integrated Octopus-client supports ACME-v2:
Octopus 2019.10.3, 2019.9.6 LTS, 2019.6.12 LTS or newer use ACME v2, which is required after Let's Encrypt retired the v1 APIs in November 2019.
Check, if it is possible to use that integrated solution.