Make sure Sponge, Spigot, LuckPerms or any other non-Forge mod/plugin/loader isn't installed! FTB Utilities doesn't work well with those. If not, make an issue or ask question on discord in #chat-and-support channel.
Read the Ranks page
You can find the permission list here
Usually you will want to run /ranks setp player <permission node here> <value here>
Poorly. It's best to not use those when if you use FTBU or not use FTBU if you want to use those mods.
Set command.name permission to true. If command is from mod, then it will have modid in it. For example, /tp will be command.tp but /spawn will be command.ftbutilities.spawn.
Command nodes will always be command.modid.commandname. For example, /home command comes from FTB Utilities mod, so it will be command.ftbutilities.home. Vanilla commands don't have a mod ID so they are just command.tp or command.stop.
If you have Aurora mod installed, you can view them from http://localhost:48574/ftb-utilities/ranks/commands in browser.