Why is my monitor not letting me select 1440p and 120hz at the same time?

Ye I have that issue to . It shows but when I tick the 120 the 4k shuts of to 1080p . I'm on insider's so I put it down to that 🤔

Im using a viotek gn27b. 144hz 1440p gaming monitor. But ive looked into it and about a year ago someone else complained about the same problem with monitor a few others and i was just seeing if thier has been a fix since then

A few things to consider....

Are you using an insider build of the xbox operating system?

And what colour depth have you selected?

Also, although monitor may support 120hz, it may only support it via display port and not hdmi.

I dont think the Xbox One S can do 1440p at 120hz, only 1080p at 120hz. at 1440p you'll only see 60hz.

Below is a little more information that might help.

120Hz is only available for 1080p and 1440p. If you change your resolution to 4K UHD or 720p, the refresh rate
will be set to 60Hz by default. If your console is set for 4K UHD and you change your refresh rate to 120Hz, your resolution will be downgraded to the highest resolution available
for your display.

To take advantage of 120Hz, you'll need an Xbox One X or Xbox One S running the Xbox May 2018 Update. This update is currently limited to a subset of users enrolled in the Xbox Insider Program, ahead of a wider release set for the coming weeks.

Your Xbox One must also be connected to a compatible TV or monitor, which supports refresh rates over 60Hz. High refresh rates must be supported over HDMI 2.0, due to limitations of the Xbox One hardware.

Constraints of HDMI 2.0 also means 120Hz support cannot be used in conjunction with 4K resolution. And bandwidth limitations mean 1440p or 1080p resolution must be used, cutting a few pixels for those extra frames.

But even with the feature, don't expect system-wide improvements. Many Xbox One titles are developed with hardcoded frame-rate limits, set between 30Hz and 60Hz. While some games are developed with uncapped frame rates, expect many games to see no changes.
We recommend researching your favorite titles for details on frame rate settings.

Because of this resolution drop, the feature isn't recommended for people many with 4K displays. With few games offering 120Hz support on the console, the benefits are limited. But for 1440p or 1080p users, there are advantages.

How to enable Xbox One 120Hz support

To get started with high refresh rates on Xbox One, you'll need to enable the feature on your console. After installing the latest update, follow these steps:

1.Navigate to the Settings app on your Xbox One.

2.Choose the Display & sound tab

3.Select the Video output tile.

4.Open the Refresh rate drop-down menu.

5.Choose 120Hz to increase your refresh rate. (Your console should now output a 120Hz signal.)

That monitor supports 120hz ONLY at 1080p, and 60hz at 1440p according to the companies website. That would be why you cant select 120hz at 1440p

"With our monitors, you can run 120Hz at 1080p resolution. Or you can rock a pixel-perfect 1440p resolution at a 60Hz refresh rate. Even knowing this, you’re now able to enjoy buttery smooth gameplay
on one of the most popular game consoles of all time — thanks to FreeSync support"

From viotek website