DIAGNOSIS You need to remove the monitor's default stand bracket first. It covers the 100x100mm VESA pattern. The process isn't intuitive.
FIX Lay the monitor face down on a soft, flat surface. Firmly grip the stand neck where it meets the monitor. Pull it straight DOWN (toward the bottom of the monitor), do not twist. It is a pressure-fit clip. It will require significant force and make a popping sound when it releases. Once the stand is off, you'll see the four VESA screw holes and a plastic cover in the center. Attach the mounting plate from your arm using the four M4 screws (usually provided with the arm, not the monitor).
IF THAT DIDN'T WORK You might be using screws that are too long. The screw wells in the monitor are shallow. Using standard VESA screws longer than 10mm can damage the internal panel. Verify you are using M4 screws with a length of 6mm to 10mm. Do not overtighten.
HEADS UP The monitor is relatively light, but its curved screen makes the center of gravity different. Ensure your monitor arm is rated for the weight and that you tighten all arm joints securely to prevent sagging or tilting once mounted.