@DX if you are using Setup -> Import -> import CSV, then in Netsuite fields you have memo listed twice. Fist time under "Journal Entry" - this one goes onto header. Second time is in section called "Journal Entry Line", this one will put memo on the line

yeah but how do I make it so that NS will map the memo line in my csv with the field mapping in NS automatically?

Header memo is usually called Memo(main) if you name your column accordingly then it should map correctly. Otherwise, map it manually then save the template and reuse.