If you have a High Speed 4K HDMI cable. What is the longest it can be for Xbox 1X?

20m is the big max to take advantage of the cable without signal degradation.

I had a 50 foot HDMI 2.0 braided one for my Xbox One X and it worked fine in 4K HDR.

20 meters is recommended to be before slight quality issues exist. However, if you run 45-50ft it should at least still detect just will not be perfect quality most likely. Are you able to try using a different 45ft cable to see if that is the issue?

50 feet seems like a long reach to me. Like other have said cables are rated very differently. I bought a new 4k cable that is like 6 foot long and have zero issues with it. But no matter how long cable is it should still recognize signal from the Xbox.
You might want to consider a better new cable for your set up even if it is 50 feet long if you get the right equipment everything should work out for you. Good Luck!!!

I have switched up my cable a few times because of this reason. I had problems with a 20 foot, and a 15 foot cable cutting out. I am now using a 6 foot cable and it works great.