According to specs, it does support 8GB of RAM.
https://www.cnet.com/products/dell-latitude-e6410-14-1-core-i5-520m-windows-7-pro-2-gb-ram-250-gb-hdd-series/
It could be a compatibility issue. You purchased DDR3L, and the DDR3 SDRAM is on the spec sheet. Sometimes RAM is finicky.
As for the...