Help with Data Validation issues

Hi All

i have a large worksheet which I use to record my training activities in. On Column I - this is the Expiry date to which I populate using a Vlookup formula to another predefine list on another worksheet. The only problem I have now is that I get the following message as it will not allow me to put this formula in (=H1112+VLOOKUP(E1112,List!$E$2:$F$21,2,FALSE) which works perfectly in row 2 - 1000. anything over this I get this message:-

MsgBox "Data validation on selected cells have been overridden. Changes have been undone. "Invalid Operation"

I have tried removing the data validation from the selected cells but it doesnt seem to work and therefore is there any way of telling what is contained in the VBA that prevent this change?

Any help appreciated on this matter.

Issue has now been resolved