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.