Sorry… I didn’t realize it was my turn to bat… 
I guess it’s this post that you’re following?
Since you’ve typo’ed the "chmod +x … ” command in the title, you really need to carefully follow each step (and avoid any typo’s) in that post. Each command is case-sensitive and must be executed successfully (returns to command line prompt without error messages) before the next is attempted.
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I thought it was pretty step-by-step already but I’ll try again here…
Step 1: From the Linux “Terminal”… you first install a necessary library with (omit the quotes)
“sudo apt install libpulse-mainloop-glib0”
Acknowledge any prompts to allow the command to complete. You may have to reboot your Chromebook afterward.
Step 2: Download the Lightburn .run file (9.16 was current when I did this)… it will go to the Downloads directory. Use the “Files” (file manager) to move (or copy) the .run file to your Linux home directory (/home/<your_name>) in order to change permissions… the Downloads directory is still out in the “cloud” somewhere and won’t allow changing the permissions from there.
Step 3: From the Linux “Terminal” again, type (without quotes)
“cd”
to insure you’re in your home directory (/home/<your_name>) and then
“chmod +x LightBurn-Linux64-v0.9.14.run”
to actually change the permissions and make it executable. Use the exact filename of the .run file you downloaded and make sure to type it exactly as shown (“+x”, not “X+”).
Step 4: Again, in the terminal run execute the file using
./LightBurn-Linux64-v0.9.14.run
Make sure to use the exact filename of the file you downloaded. Once that completes you should have a Lightburn icon in your menus.
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I hope this helps. If you aren’t comfortable using a Chromebook and/or Linux, this may be very confusing and/or cryptic but I really don’t know how to be any more explicit than this. Hopefully someone may be able to jump in and make things clearer.
Good luck!