I'm trying to upgrade the RAM on my ancient OptiPlex 780 MT (2005-2009 I think) from 8G to 16G. I got 4 sticks of Crucial 4GB DDR3L 1600 MHz UDIMM (see link below) but the machine wouldn't boot and flashed the simultaneous 3+4 diagnostic lights (indicating memory config failure).

https://www.bhphotovideo.com/c/product/1170948-REG/crucial_ct51264bd160bj_4gb_1_x_4gb.html/c/product/1170948-REG 

Do I not have the right RAM? If not, which one do I need?

Or could it be my BIOS, which is Dell A03 ?
Thanks for any help!