Problem upgrading PHP on my server

Hi, when I try to install PHP 8.0 on my server I get an error. This is the error I'm getting:

Hit:1 http://archive.ubuntu.com/ubuntu bionic InRelease
Hit:2 http://ppa.launchpad.net/chris-lea/redis-server/ubuntu bionic InRelease
Hit:3 http://ppa.launchpad.net/nginx/development/ubuntu bionic InRelease
Hit:4 http://ppa.launchpad.net/ondrej/apache2/ubuntu bionic InRelease
Hit:5 http://ppa.launchpad.net/ondrej/php/ubuntu bionic InRelease
Get:6 https://repos.sonar.digitalocean.com/apt main InRelease [4875 B]
Hit:7 http://mirrors.digitalocean.com/ubuntu bionic InRelease
Hit:8 http://mirrors.digitalocean.com/ubuntu bionic-updates InRelease
Hit:9 http://mirrors.digitalocean.com/ubuntu bionic-backports InRelease
Get:10 http://security.ubuntu.com/ubuntu bionic-security InRelease [88.7 kB]
Err:6 https://repos.sonar.digitalocean.com/apt main InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY DE88104AA4C6383F
Reading package lists...