add another ram stick

[meedo.barca10]

if i have a 1Rx16 ddr4 ram stick in laptop can i mix it with 1Rx8 and other specs of both sticks are same?

 

[thx1138v2]

If you are using an Intel processor and it supports "Flex Channel RAM" that might work. Otherwise, probably not and definitely not on an AMD system.
https://www.avadirect.com/forum/Message/89-Single-Channel-Dual-Channel-and-Flex-Channel-RAM/

Your best option is to go to the manufacturer's web site and find another 1X16Gb module for that machine.