Could not build key. (Error 3250)
When building a primary index, the Microsoft Access database engine could not build a primary key. Make sure the key fields are named properly and that there are no duplicate records based on this key.
This error can occur when you use the Seek method and pass it a value for a field that is not part of the index. For example, suppose you want to use the Seek method on a Recordset whose current index uses the LastName field of the underlying table, and you write it this way:
The Microsoft Access database engine will try to construct a primary key from two fields, but because one field is indexed, the attempt will fail and this error results.