Hi there, I’m trying to run armbian on cs918 plus tv box with rk3288 cpu. I am using image from this site for xt-q8l-v10 boards. It boots into uboot and shows Ethernet error “ethernete@ff290000 Waiting for PHY auto negotiation to complete.

.. TIMEOUT” and after showing this error around ten times uboot just stucks and shows arrow. So how can i fix this error. Thanks in advance

Your title is misleading.  You don't want to disable Ethernet.  What is happening is that uboot is failing to boot and as a last resort trying a network boot.

Your problem is that you can't just take a build for a different box and expect it to work for your box.

In general TV boxes are not supported by Armbian.  You need to search the forums or the wider Internet to see if anyone has had any success with installing on your specific box and how to do that.  If no one has them you always have the option of adding the support for your box using the Armbian build framework, but that is a daunting task if you are not already familiar with porting Linux and uboot to sbcs/tvboxes.