Query doesn't work with AVG function, how to use AVG in SQL?

Based on your above question, I have created both the table on my local pc using the script

//1.    Student Table

CREATE TABLE [dbo].[student](

                [std_id] [int] NOT NULL,

                [std_Name] [nvarchar](50) NOT NULL,

                [std_cont] [nvarchar](50) NOT NULL,

  )

 

//2.    Grade Table

 

CREATE TABLE [dbo].[Student_grade](

[std_id] [int] NOT NULL,

[Sub_NAme] [nvarchar](50) NULL,

[Sub_grade] [int] NULL,

) 

and tried executing your query after removing "Where" clause

SELECT   [std].[std_id], 
          [std].[std_name], 
          [std].[std_cont], 
          Avg([rate].[Sub_grade]) AS [std_marks] 
 FROM     [student] [std] 
 JOIN     [student_grade] [rate] 
 ON       [std].[std_id] = [rate].[std_id] 
 GROUP BY [std].[std_id], 
          [std].[std_name], 
          [std].[std_cont]

and it works for me with AVG function as needed, here is the screenshot