Error received when trying to add to a drop down list

[Noel]

I am trying to add more options to a drop down list in an excel spreadsheet
and I am receiving the following error:
"The vaulie you entered is not valid. A user has restricted values that can
be entered into this cell."

This spreadsheet was created by a coworker and then given to me. I believe
they copied the information from another spreadsheet so could it be possible
that some protected information was copied over?

Thanks