Unexpected Error when installing Mac OS Mojave

Hello,

I work in a college and over the summer I've started to update our 14 macs to Mac OS Mojave. (Due to a 8bit/16bit software issue we couldn't update sooner).

12 of the macs have completed the installation without any issues at all.

2 macs however get to the last 5 seconds of the installer and report an 'unexpected error' and then exit the installation.

I'm upgrading from Sierra to Mojave, although on one of the macs I installed High Sierra first as a 'stop-gap' to see if that would then let me install Mojave but this didn't work either. I have already tried installing through Safe Mode and checked that the time is correct on the Mac. I can't think of anything that has been done on these macs that would be different to the other 12 which were successful.

Can anyone suggest why this is happening? I need to get these two updates ASAP so that students can begin coursework on them from W/C 9th September.