Column <Columnname> in table <Tablename> participates in index <Indexname>. Columns with a datatype of <Datatypename> cannot participate in indexes or primary keys. (Visual Database Tools)
You have attempted to change the datatype of a column that is part of an index or a primary key. Indexes and primary keys cannot be created for columns with datatypes such as TEXT or IMAGE. To change the datatype, first remove the index or primary key.
For more information see Creating Indexes (Database Engine), How to: Create Unique Indexes (Visual Database Tools), and How to: Create Primary Keys (Visual Database Tools).
See Also
Concepts
Error Messages in Visual Database Tools