memory problem with iMac late 2015
Your model iMac uses 204-pin PC3L-14900 (1867 MHz) DDR3 SO-DIMM memory chips, so you need to ensure that any chips you use meet this criteria. 

You can go for faster memory (so long as everything else matches), but note the Mac won't use it at those faster speeds. I mention it just in case you can't find compatible chips at the correct speeds. Also, there's no mention of voltage in the official specs, but the 1.35v is fine. 

As an aside, your iMac can actually use up to 64GB RAM (not just the 32GB officially supported by Apple).