The server's SSL certificate could not be validated

Hi experts. A newbie here. Three issues on my setup. I setup a Bitwarden server on Ubuntu. At first, I used self-signed certificate, it works, however with limited capabilities. Now, I want to test it on different users.

1st) I need SMTP to connect and validate the user by email. I got this red error: An error has occurred. An unexpected error has occurred. I have configured the global smtp settings to connect to port 587 and I am able to connect via telnet.

I check the logs:

[Error] An error occurred while attempting to establish an SSL or TLS connection.

The server’s SSL certificate could not be validated for the following reasons:
• The server certificate has the following errors:
• unable to get local issuer certificate

MailKit.Security.SslHandshakeException: An error occurred while attempting to establish an SSL or TLS connection.

2nd) It errors on me too when I access the admin portal, https://vault.domain.com/admin. If I can fix the SSL cert, this should also be fixed (I think).

3rd) I tried to fix the above mentioned error by following the documentation here: Certificate Options | Bitwarden Help Center

However, it was not successful, I miss this part of the command where it ask for a full path from, I wonder what path to put in there.

docker run -i --rm --name certbot -p 443:443 -p 80:80 -v <Full Path from / >/bwdata/letsencrypt:/etc/letsencrypt/ certbot/certbot certonly --email [email protected] --logs-dir /etc/letsencrypt/logs

I hope to get an idea of the fix and you can refer me to any article that can fix this error. Thank you so much in advance.