March 19 (Reuters) - Canada’s main stock index opened lower on Friday, pressured by weakness in bank stocks and data showing domestic retail sales fell in January hurt by coronavirus-led restrictions. * At 9:35 a.m. ET (1335 GMT), the Toronto Stock Exchange’s S&P/TSX composite index was down 21.24 points, or 0.11%, at 18,815.23. (Reporting by Amal S in Bengaluru; Editing by Amy Caren Daniel)