All queries combined using a UNION, INTERSECT or EXCEPT operator must have an equal number of expressions in their target lists.

Error:

Msg 205, Level 16, State 1, Line 1
All queries combined using a UNION, INTERSECT or EXCEPT operator must have an equal number of expressions in their target lists.

Reason:
The select statements in the clause do not have the same number of columns.

Fix:
Make sure that all the starements using UNION, INTERSECT or EXCEPT operator have same number of expressions(columns).
Use column names instead of using * in the select list.

Sample example code

Before:

SELECT 1
UNION
SELECT 2,3

After:

SELECT 1,NULL
UNION
SELECT 2,3