While it's correct that the system has just one memory slot as noted by others, the CPU you have controls the speed of the memory -- if you're seeing 2400 now, you will see that even if the RAM is capable of faster operation.  The likely reason Dell offers 2666 is because it's been around long enough that no one makes 2400 any longer -- DDR-4 has long since moved on from there (and is being rapidly replaced by DDR5 in new systems now).  
Hi? I would to ask, is my Inspiron support DDR4 2666Mhz 16GB?  
Dell Inspiron 7472 RAM Compability