If you've used Disk Utility in Mojave APFS you know it's become much more complex than HPFS days. Recently I tried running it from the Recovery start and got a Disk Locked error that said I needed to wipe the drive and reformat. I couldn't find any fix for this on Google.
This did fix it:
1. I rebooted into macOS and ran Disk Utility from there.
2. I rebooted back into the Recovery mode and this time the disk was not locked.
I don't know what did the trick. Maybe all I had to do was go back into normal boot and then back to Disk Utility, but I think I tried that and it wasn't sufficient. I had to run Disk Utility from macOS.
I suspect a safe boot might have worked too. Anyway, the point is, if you're stuck and you find this post it's fixable.
PS. This isn't about mounting an encrypted disk.