Can I mix an 8GB and 16GB RAM stick in my computer?

Yes, it will almost certainly boot and run at 2133MHz. The system uses “flex mode,” treating the first 16GB (8+8 from each stick) as dual-channel and the remaining 8GB as single-channel. You probably won’t notice the difference unless doing memory-heavy tasks. Just ensure both sticks are the same voltage (1.2V for standard DDR4) and check your motherboard’s QVL list if you’re worried.