Ubuntu 18.04.6 LTS Released To Correct Broken Install Media
This extra Ubuntu 18.04 "Bionic Beaver" LTS point release stems from the install media breaking due to key revocation. The issue stems from the BootHole vulnerability and the keys used by Ubuntu having been revoked and thus needing to issue Ubuntu 18.04.6 LTS with new keys.
Hopefully by now you have migrated to Ubuntu 20.04 LTS or one of the newer non-LTS releases, but for those sticking to Ubuntu 18.04, the 18.04.6 install media is out today for x86_64 and AArch64 in both desktop and server ISOs. If not running UEFI SecureBoot, the prior Ubuntu 18.04 LTS install media should still work fine.
Ubuntu 18.04.6 also includes the latest Bionic stable release update packages too so additional fixes are rolled into these ISOs.
The brief Ubuntu 18.04.6 LTS release announcement can be read on ubuntu.com.