A candidate cannot enter units on public holidays - why is this?

There is nothing at all on the system that would block a candidate from entering units worked on public holidays but I suspect what has occurred here is they have entered some units against a public holiday (Bank Holiday Monday for example), then used the Copy to all other working days option to fill out the rest of their timesheet:

As public holidays and weekends are not classified as 'normal' working days, the system will always clear them out when the Copy to all other working days option is used.

If they have worked on a public holiday or weekend they must edit those days separately to enter units against them after using the Copy to all other working days option (if they use this to fill out their timesheets).

The system will pick up public holidays as per your global site setting, for example if your site is set to England then all British public holidays will be detected. If you have a candidate working in a different country it will not be possible to set their assignment separately to pick up that countries respective public holidays as this setting applies to your entire ETZ site.