insert into table variable from select statement

You just need to reorder your statements. Declare the temporary table first and then do an INSERT using the SELECT as the operator that will provide the values for the INSERT.

For example:

 DECLARE @Index_Table TABLE( Database_id smallint null
  , Object_id int null
  , Index_id int null
  , Name sysname not null
  , Type tinyint null
  , Type_desc nvarchar(60) null
  , Avg_fragmentation_in_percent float null
  , Fill_factor tinyint null
  , RowId int identity(1,1))
  
  INSERT @INdex_Table (Database_id
     , object_ID 
     , Name
     , Index_ID
     , Type
     , Type_desc
     , Avg_fragmentation_in_percent
     , Fill_factor)
 SELECT ps.database_id
 , ps.object_ID
 , so.name
 , ps.index_id
 , b.type
 , b.type_desc
 , ps.avg_fragmentation_in_percent
 ,b.fill_factor
 FROM sys.dm_db_index_physical_stats (DB_ID(), NULL, NULL, NULL, NULL) AS ps
     INNER JOIN sys.indexes AS b ON ps.OBJECT_ID = b.OBJECT_ID
     INNER JOIN sys.objects so ON b.object_id = so.object_id AND ps.index_id = b.index_id
 WHERE ps.database_id = DB_ID() 
 ORDER BY ps.OBJECT_ID 
 
 SELECT * FROM @Index_Table
 GO

Hope this helps!!