In the latest releases, Ubuntu performs signature check for kernel modules before they are installed. Hence you need to load public key of kernel module into Ubuntu firmware so that it recognizes module's signature.
Otherwise, here is the steps to disable Secure Boot in Ubuntu without reinstalling system. This is applicable especially if you have installed as VM.
Identify if secure boot is enabled or disabled on Ubuntu
$ sudo mokutil --sb-state
The result of above command shows the status of SecureBoot if enabled or disabled.
#1 Install mokutil to disable secure boot
$ sudo apt install mokutil
$ sudo mokutil --disable-validation
Note - double hypen before disable-validation
When asked for password, specify 12345678. For convenienceDo not create a complex password, as you have a trick in the following screen.
#2 Press Any key in Shim Signed Key Management
You get 10 seconds to respond and follow the screenshots to disable secure boot.