Diagnosis
You can, but it's not straightforward. The LG 27MQ400-B does have VESA mount compatibility (100x100mm), but the screws and the interface are hidden behind the stock stand.
How to Fix It
To access the VESA holes, you need to remove the default stand. Lay the monitor face down on a soft, flat surface. Firmly grip the stand neck where it meets the back of the monitor. There's a release mechanism—you need to press a small plastic tab (usually located at the top of the junction) inward while pulling the stand straight down and away from the monitor. It should detach. Once off, you'll see the four VESA screw holes and a plastic cover plate in the center. Remove that plate, and you can attach your VESA arm using the appropriate screws (typically M4 x 10mm length, but check your arm's instructions).
If That Didn't Work
If the stand won't budge, don't force it. Double-check for a locking lever or button. There are official LG tutorial videos for this process—search "LG monitor stand removal" for a visual guide. Using the wrong screws or over-tightening can damage the internal threading.
Heads Up
The monitor is very light, so most arms will handle it fine. Once the stand is off, there's no way to re-attach it without the VESA bracket, so make sure you're committed to the arm or store the stand safely.