I can’t update/upgrade. How do I fix this?

root@starfish:~# apt-get update && apt-get upgrade -y
Hit:1 http://mirrors.digitalocean.com/ubuntu bionic InRelease
Err:2 https://repos.insights.digitalocean.com/apt/do-agent main InRelease                               
  400  Bad Request [IP: <SNIP> 443]
Hit:3 http://mirrors.digitalocean.com/ubuntu bionic-updates InRelease                                   
Hit:4 http://mirrors.digitalocean.com/ubuntu bionic-backports InRelease                                 
Get:5 http://security.ubuntu.com/ubuntu bionic-security InRelease [88.7 kB]                             
Hit:6 http://ppa.launchpad.net/ondrej/php/ubuntu bionic InRelease                                       
Hit:7 http://ppa.launchpad.net/wordops/nginx-wo/ubuntu bionic InRelease                                 
Get:8 http://mariadb.mirrors.ovh.net/MariaDB/repo/10.3/ubuntu bionic InRelease [6,265 B]                
Reading package lists... Done                      
E: Failed to fetch https://repos.insights.digitalocean.com/apt/do-agent/dists/main/InRelease  400  Bad Request [IP: <SNIP> 443]
E: The repository 'https://repos.insights.digitalocean.com/apt/do-agent main InRelease' is no longer signed.
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.