ERROR_MESSAGE returns the error message cause in SQL Server statement. The Scope of ERROR_MESSAGE is within catch block of try catch. If called outside the catch block it will return NULL.

Example

BEGIN TRY   
    SELECT 'Hitesh'/5 as Value;
END TRY
BEGIN CATCH
    SELECT ERROR_MESSAGE() AS ErrorMessage;
END CATCH;
 SELECT ERROR_MESSAGE() AS ErrorMessage;

Result
ErrorMessage
Conversion failed when converting the varchar value ‘Hitesh’ to data type int.

When called outside the catch block
ErrorMessage
NULL