Hi @koushik!
For the 2nd datatable you need to add a empty column first, change it to the first position and then do the merge.
To do this, use “Add Data Column” activity to add a column in the 2nd datatable and then follow the instructions on the solution of this post: Add Column and make it as the FIrst Column - #2 by Jan_Brian_Despi to make it the first one. Finally, do the merge with the 1st datatable.
Hope it helps,
Best Regards