To Amazon and LG to see what they say to do next. For the TV you check if the firmware is current and same for the Amazon stick. On some sets you have a LEGACY HDMI setting that must be turned off. I didn't check your model's menus since these are the basics I run into on this.
There are folk that put the stick or box on a HDMI extension cable. Don't do that.
I wanted to purchase and stream a 4k video from Amazon Prime video using my Amazon Fire Stick 4k. An error came up prior to purchase stating my TV does not have compatible HDMI port with HDCP 2.2. capability. However, when I reviewed the specs on the tv (LG 50UK6500AUA), all 4 of the HDMI ports are claimed to be HDCP 2.2. Can someone help me understand and/or fix?