Ram is sold in kits for a reason.
A motherboard must manage all the ram using the same specs of voltage, cas and speed.
The internal workings are designed for the capacity of the kit.
Ram from the same vendor and part number can be made up of differing manufacturing components over time.
Some motherboards, can be very sensitive to this.
This is more difficult when more sticks are involved.
Adding a second similar stick may work.
But, you may have to fiddle in the bios with ram settings and voltage.
Then, there is no guarantee that you can get it all to work.
Your single stick of ram is running in single channel mode, effectively at 3000 speed.
That is why a dual stick kit will be much faster.
A 2 x 16gb kit of DDR5 6000 speed ram will be about $100.
Most any kit will work, but to be certain, look for one that is explicitly supported for your motherboard.
Your motherboard will be identified in the motherboard tab when you run cpu-Z.
Ram vendors have support apps where you enter your motherboard make/model and you get a list of supported ram.