There is no such file on my Beaglebone Rev A5, and I suspect the same is true for your A6.
sudo -i ls /sys/kernel/debug/omap_mux/uart*
You are getting the message because the kernel won’t allow you to create a new file in this directory.
I believe you want to use /sys/kernel/debug/omap_mux/lcd_data13. It’s confusing, but the files are named for their mux mode 0 functions by the software.
Once you find the right file, I suspect you need to echo something different than that to it, also. I have never tried setting bits more significant than the least significant 6 bits (corresponding to hex numbers 3F or less), and I’m not sure what that will be doing.