Mono Headphone Sound

Why am I only hearing mono sound from my headphones? The connection is using an xlr to the 1/8 adaptor on the PMXU63BT mixer?

An XLR connection is mono. You need two for stereo.

The 1/8" (3.5mm connector) is probably stereo so you might need a couple of adapters to get everything wired correctly.

Maybe explain what you’re doing with a little more detail. You may need 1/4" plugs for a line-level (or headphone level) connection. The XLR inputs are high-gain microphone inputs.