If I say sudo apt update, I got this error:
 

Hit:2 http://ports.ubuntu.com bionic InRelease
Hit:3 http://ports.ubuntu.com bionic-security InRelease                        
Hit:4 http://ports.ubuntu.com bionic-updates InRelease                         
Hit:5 http://ports.ubuntu.com bionic-backports InRelease                    
Ign:1 https://apt.armbian.com bionic InRelease                             
Err:6 https://apt.armbian.com bionic Release                               
  Certificate verification failed: The certificate is NOT trusted. The certificate issuer is unknown.  Could not handshake: Error in the certificate verification. [IP: 193.40.103.96 443]
Reading package lists... Error!
W: http://apt.armbian.com/dists/bionic/InRelease: No system certificates available. Try installing ca-certificates.
W: http://apt.armbian.com/dists/bionic/Release: No system certificates available. Try installing ca-certificates.
E: The repository 'http://apt.armbian.com bionic Release' no longer has a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: Encountered a section with no Package: header
E: Problem with MergeList /var/lib/apt/lists/ports.ubuntu.com_dists_bionic_main_binary-armhf_Packages.lz4
E: The package lists or status file could not be parsed or opened.

However, if I open https://apt.armbian.com on my computer, TLS connection is just fine. But on a Armbian (Ubuntu 18.04.2 LTS), I got this:

curl https://apt.armbian.com
curl: (77) error setting certificate verify locations:
  CAfile: /etc/ssl/certs/ca-certificates.crt
  CApath: /etc/ssl/certs

Any idea why this happened?

No?

Just do sudo apt update again, and maybe again until it's fine. There seem to be problems with all distro's. Not only armbian.
I often need to do it 3 times before it's ok.