This is the error
BLAST Database error: No alias or index file found for nucleotide database [nr] in search path [/home/aj3/Desktop/h/database::]
what should I do? do I have to use any other commands prior to this one to make the database ready or what?
also please don't ask two identical questions, the other has been deleted
It did not give you the "not found database" error: the error is "No alias or index file found" - as you state yourself.
Difficult to diagnose the issue without more information, but one possibility is that you have the wrong database. "nr" usually refers to non-redundant protein, but your error refers to a nucleotide database.