Open Manage | Database | FIelds
Find that field, gpublication, and double click it's field definition.
Click the validation tab
Clear the "not empty" check box
That clears the error, but you may need to investigate why this field can't be empty.
Fields with a g at the front are often set by the developer to be a global field. Global fields behave differently when the file is hosted over a network.
See this article for more on the subject:
I had a similar issue and found the problem, at least what it was in my database:
Basically check that the next auto assigned ID in the portal ID record is higher than the record with the highest ID number.
The error is thrown because an ID is assigned that already exists, which causes a validation error if you have "Unique" validation selected.
My error was caused when I imported data which contained higher ID's than the field was defined to assign as the next ID.
The problem is that the error text does not directly convey that it is the validation check which fails. In my case it was the "Unique" validation that failed.
The error text misdirects your attention, by stating that the field requires a value (making you think the ID is empty, when it is not) and that the field is not available on the layout (which in this case is irrelevant as the ID field is autogenerated).
What the error SHOULD say is: "The portal field "xxx::yyy_ID" failed validation as it is not Unique."
That would direct your attention to the actual error.