monitor input not supported

what specifications must a monitor have to be used with the tk1?
my hdmi monitor shows “INPUT NOT SUPPORTED”

It sounds like it is the monitor itself saying “input not supported”, that it is unlikely the Jetson is sending that information. I once had a monitor on a system work great for a long time, and then one day it gave me that same message you posted; I’d thought the video card died, but instead it was the port on the monitor itself (I verified it worked with the same video card on a different monitor connector and that the same video card worked with the original connector on a different monitor).

But, for Jetson to support a monitor, the monitor basically needs to use any port except an older analogue VGA port. Almost every other port in existence has what is known as a “DDC” channel, which provides DDC and EDID data which the video system can read and query to know what the monitor is capable of. This is physically an extra wire versus the 15-pin analogue connector, and so even an adapter on a cable somewhere converting to VGA will cause failure. There are all kinds of adapters and connectors which will work, e.g., HDMI-to-DVI, because they have this DDC channel.