Mary -
Were able to find a solution for this? If not, you'll need to convert the field to a Formula Text field rather than a Formula Numeric field. Since your formula is a Formula Numeric, you can't have any text options for the 'Else' part of your formulas. OR If you would just like to produce errors for your users, you can also use this formula in the Business Rules section for your table:
------------------------------
Blake Harrison
bharrison@datablender.io
DataBlender - Quick Base Solution Provider
Atlanta GA
404.800.1702 /
http://datablender.io/------------------------------