If you see Input Not Supported Monitor error on your Windows 11/10 PC, you can apply any of the solutions in this post to resolve the issue.

Dell U2312HM The current input timing is not supported by the monitor display. Please change your input timing to 1920×1080@60Hz or any other monitor listed timing as per the monitor specifications.

I do not know why, but it did not like the video signal the RX 5700 XT was sending it. What makes it really odd is that monitor was on AMD's list of certified/recommended monitors.