Bot Permissions

It is highly recommended that the bot is an Administrator. This can avoid many permission problems that may occur. I never test the bot with individual permissions, only with the bot having Administrator (full permissions). Chances are, the invite (OAuth link) you used already had Administrator ticked.

If the bot is missing permissions it needs for a command, it will tell you.

I gave the bot permissions, but it still fails/does nothing!

Chances are, the bot is getting permission errors due to the hierarchy. This means that the bot's role is too low and cannot mess with people who have roles higher than the bot's role. Move the bot's role up.