DIAGNOSIS This monitor uses a 75mm x 75mm VESA pattern, but the mounting holes are recessed in a plastic housing on the back. You can't use the standard short screws that come with most mounts; you need longer screws to reach through that plastic and into the metal threading.
FIX You need M4 screws (metric, 4mm diameter). The length is critical. 1) Remove the stock stand by holding the monitor upright, pushing down on the quick-release lever at the neck, and lifting the monitor off. 2) Measure the depth from the back of the recessed hole to the start of the metal threads. You'll likely need screws that are 10mm to 12mm long. 3) Attach the VESA plate from your mount using these longer M4 screws. Do not overtighten; snug is enough. The plastic housing is just a spacer, the real strength comes from the screws biting into the metal chassis inside.
IF THAT DIDN'T WORK If your mount's plate is too large and covers the recessed area, you may need to add plastic washers or spacers (often included with better mounts) between the plate and the monitor back to bridge the gap before you put the screws in. This prevents the plate from putting pressure on the plastic.
HEADS UP Do not, under any circumstances, force the original stand's screws or any other screw into the VESA holes. You will strip the soft metal threading inside the monitor. If you're unsure, take the monitor to a hardware store and test-fit M4 screws in the electrical aisle before you buy.