FLINT 2 - LuCi-app-SQM 255 ERROR

[admon]

GL-MT6000 Cannot install SQM

Some advanced packages are missing in the GL repository. The best way, for advanced usage, would be to switch to plain OpenWrt. Then you can work with all available packages inside the OpenWrt repo. Or you could wait until GL pushed those packages into the repo as well.

That's the reason why you can't install it right now.
Switching to the op24 version of the firmware might fix this.