A subquery that is used in an expression in the place of a literal value must return only a single row and a single column. In this statement, a subquery has returned more than one row, and the database server cannot choose which returned value to use in the expression. You can ensure that a subquery will always return a single row. Use a WHERE clause that tests for equality on a column that has a unique index. Or select only an aggregate function. Review the subqueries, and check that they can return only a single row.
This error can also occur when you use a singleton SELECT statement to retrieve multiple rows. You must use the DECLARE/OPEN/FETCH series of statements or the EXECUTE INTO statement to retrieve multiple rows.