I had this exact issue and foolishly wiped the drive not realising that the security was set to only internal drives.
William Tong’s answer is great, but similar to commenter, I could still not boot into recovery with the blank Mojave’s SSD. It would prompt that it couldn’t boot from external drive and to choose a startup disk or restart.
I was able to get to recovery mode by starting to select a start up disk and then quitting with cmd-q. Took me back to recovery mode and I was able to install Mojave on the internal drive!
Sorry I can’t comment this beneath the post, I’ve only just started participating in the forum.