Gestetner-2412Z copier-error code UF

Question

Gestetner Destktop 2412Z Copier displaying error code: UF. What does it mean and how to resolve
or clear it? The copier can initialize when I turn on the popwer but
cannot copy due to error. I carried out simulation 02 and I was able to
print several copies but when I lleave simulation. UF displays again.
I'd checked the developer powder and found out that it's running out.
Does it have something to do with the error? I'll appreciate any help
or assistance you will extend to me.