MacBook Air (13 inches, 2017) connecting

MacBook Air (13-inch, 2017) - Technical Specifications - Apple Support

"Simultaneously supports full native resolution on the built-in display and up to 3840-by-2160 resolution at 60Hz on an external display, both at millions of colors

• Thunderbolt digital video output
• Native Mini DisplayPort output"

If https://www.mi.com/global/product/xiaomi-4k-monitor-a27ui/ is the monitor in question, it would seem to support a 10-bit color depth. 3840x2160 pixels @ 60 Hz with 10-bit-per-channel color exceeds the capabilities of the 2017 MacBook Air. You may need to configure the monitor for 8-bit-per-channel color / no HDR if you want to run it at 60 Hz.

Tell us about the monitor.

What brand and model exactly?

The solution will depend on the specifications and capabilities of the monitor.

Andrey_Vladimirovich wrote:

how do I set up 8bit on my monitor?

You'll need to figure that out. There are two logical places:

• On the Mac, there might be a menu with choices for "millions of colors" vs. "a billion colors"; if so, pick the first one and see if that lets you select. higher refresh rate.
• On the monitor, there may be controls for that in the monitor menus somewhere. I don't see an online manual – so you will have to play around to find out what controls are in the monitor menus.