During my first test-run of IncusOS in an Incus VM, I got the same “Secure Boot is disabled” issue at the beginning of IncusOS install when using this parameter. Changing it to true made the installation proceed.
I brushed it off as something unimportant as I was following Trying out Incus OS , but reading this thread makes me realize this shouldn’t have happened… I have no clue so as to why it didn’t work. I’m on Arch x64 if it matters.